This was the least challenging Genius for a while, but it was an enjoyable solve, nonetheless. Once you solved the across clues, you were left with a fairly straightforward anagram, and the down clues weren’t overly difficult.
1 B(oleyn)-ITCHY – OFF
4 KESTREL – hidden in “disliKES TRELlis” – GARDEN
9 DEAUVILLE – (alive duel)* – ultra-fashionable French resort – AFTER
10 R.A.-DIX – a base number in mathematics – the Dix referred to is Otto Dix (1891-1961), a German expressionist painter – NUCLEAR
11 org. + AN – ORIGINATED
12 INCUMBENT – (NBC minute)* – ONE
13 EX(PLO)IT – (lop)* in EXIT (“way out”) – HEAD
15 PUP-ATE – NEGLECTING
17 INGEST – homophone of IN JEST – INSECT
19 STIR((Ridge)UP – MCKINLEY
22 STAGE DOOR – (Dorset ago)* – YEARS
24 ACT ON – ETHNIC
26 PIG-G(a)Y – AGRICULTURISTS
27 A-MP(h)-LIFIER ((flier I)*) – TURBO
28 NO ENTRY – (one)*-N-TRY – VEHICLE
29 ANTHEM – (the man)* – NEW So, taking the first letter of each extra word and rearranging them, we get the phrase “HAVING ONE TOO MANY“, which can be inserted at 3dn and 18dn.
The best of the DOWN clues:
1 BEDSORE – nice use of two different meanings of “lie”
4 KNEECAP – double definition
5 SCRIM – “Mass crime” with the letters of the word “same” removed
6 RED SETTER – homophone of “read” and Paul the Guardian crossword setter
8 PL(I)ANT
16 PLICATION – (in coal pit)*
20 PANGRAM – a sentence with all the letters of the aplhabet in it.
21 US OPEN – (up nose)*
25 T-HIGH – “half a lap” is inspired