Acid makes expert twitch (6)
I believe the answer is:
acetic
'acid' is the definition.
(acetic acid is a kind of acid)
'expert twitch' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'twitch' becomes 'tic' (tic is a kind of twitch).
'ace'+'tic'='ACETIC'
'makes' acts as a link.
(Other definitions for acetic that I've seen before include "Colourless pungent acid" , "Type of acid derived from vinegar" , "Like vinegar's acid" , "Saucy'" , "tart" .)