Acid complaint by an expert (6)
I believe the answer is:
acetic
'acid' is the definition.
(acetic acid is a kind of acid)
'complaint by an expert' is the wordplay.
'complaint' becomes 'tic' (a tic is a medical complaint).
'by' means one lot of letters go next to another.
'an expert' becomes 'ace' (ace can informally mean very good).
'tic' after 'ace' is 'ACETIC'.
(Other definitions for acetic that I've seen before include "Vinegar-like" , "sort of acid" , "in relation to acid" , "tart" , "Saucy'" .)