Vinegary expert with a nervous twitch (6)
I believe the answer is:
acetic
'vinegary' is the definition.
(I've seen this in another clue)
'expert with a nervous twitch' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'with' is a charade indicator (letters next to each other).
'a nervous twitch' becomes 'tic' (involuntary twitch in the body).
'ace'+'tic'='ACETIC'
(Other definitions for acetic that I've seen before include "Like vinegar's acid" , "tart" , "Saucy'" , "Colourless pungent acid" , "in relation to acid" .)