Line adopted by all in complaint (5)
I believe the answer is:
bleat
'complaint' is the definition.
(I've seen this in another clue)
'line adopted by all in' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'adopted by' is an insertion indicator (I've seen this in other clues).
'all in' becomes 'be at' (synonyms).
'l' put inside 'beat' is 'BLEAT'.
(Other definitions for bleat that I've seen before include "Sound of sheep or goat" , "kid makes such a noise" , "Cry of animal" , "Cry of sheep or goat" , "Cry (like a sheep?)" .)