The "A" of LGBTQIA+, for short (3)
I believe the answer is:
ace
'the a of lgbtqia for short' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ace that I've seen before include "Very good; playing card" , "Expert - winning serve" , "One in a suit" , "Tennis serve that the receiver is unable to reach" , "first-class" .)