Smooth and fairly nicely written. Good stuff!

Across

1

Artificial marshland maintained by American (5)
BOGUS — BOG (marshland) US (American)

4

Military headquarters are keeping my tax form (4,4)
ARMY BASE — ARE outside MY BAS (tax form)

9

Surge in irregular income cost beautician (8,7)
COSMETIC SURGEON — SURGE inside anagram (irregular) of INCOME COST

10

Lustful one tore back and front of clothing (6)
EROTIC — reversal (back) of I (one) TORE, C (first letter of clothing)

11

A loan listed in coach's schedule of events (8)
CALENDAR — A LEND (loan) inside CAR (coach)

12

Business leaders converted joint (10)
DEALERSHIP — anagram (converted) of LEADERS, HIP (joint)

15

Viral infection caught by veteran (4)
COLD — C (caught) OLD (veteran)

17

Ocean mammal ultimately? (4)
SEAL — SEA (ocean) L (last letter of mammal), &lit

18

Set catalogued out of order (10)
COAGULATED — anagram (out of order) of CATALOGUED

21

A record somehow broken by a cycling competition (4,4)
ROAD RACE — anagram (somehow) of A RECORD with A inside (broken)

22

Swallow most of cocktail (6)
MARTIN — MARTINi (cocktail, without last letter)

25

The ill cattle sit awkwardly with minor physical activity (6,9)
LITTLE ATHLETICS — anagram (awkwardly) of THE ILL CATTLE SIT

26

Head of parish, accomplished, delivered a sermon (8)
PREACHED — P (first letter of parish), REACHED (accomplished)

27

Observe dandy down and out of sorts (5)
SEEDY — SEE (observe) DandY without and

Down

1

Neck broken in bad spot at the rear (4-3)
BACK-END — anagram (broken) of NECK inside BAD

2

Puff served with duck and relish (5)
GUSTO — GUST (puff) O (duck)

3

Exhibit mutated mice inside small pen (8)
SPECIMEN — anagram (mutated) of MICE inside S (small) PEN

4

Spy group is set up following a leader of operations (4)
ASIO — reversal (set up) of IS after A, O (first letter of operations)

5

Difficulty getting on communication system (9)
MESSAGING — MESS (difficulty) AGING (getting on)

6

Fringe bishop joining religious group (6)
BORDER — B (bishop) ORDER (religious group)

7

Powered vessel passed water snake in strait (9)
SPEEDBOAT — PEED (passed water) BOA (snake) inside ST (strait)

8

Half of snakes in the sun too long may be caught (6)
SNARED — SNA (half of snakes) RED (in the sun too long)

13

Type of gypsum a research centre rates poorly (9)
ALABASTER — A LAB (research centre), anagram (poorly) of RATES

14

Small, blue train is not a fast mover (9)
SLOWCOACH — S (small) LOW (blue) COACH (train)

16

Viewing spots are lost at sea after closing of reef (8)
FLOATERS — anagram (at sea) of ARE LOST after F (last letter of reef)

17

Walk leisurely to turn at end of street (6)
STROLL — ROLL (to turn) after ST (street)

19

Limits of democracy threatening regime? (7)
DYNASTY — DY (outside letters of democracy) NASTY (threatening)

20

Crane shifted gold bar, dropping down initially (6)
BROLGA — anagram (shifted) of GOLd BAR without d (first letter of down)

23

Tense religious practice is uninspired (5)
TRITE — T (tense) RITE (religious practice)

24

Combined group in church addressed African nation (4)
CHAD — hidden in churCH ADdressed