Sour expert with a facial twitch (6)
I believe the answer is:
acetic
'sour' is the definition.
(I've seen this before)
'expert with a facial twitch' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'with' says to put letters next to each other.
'a facial twitch' becomes 'tic' (I've seen this before).
'ace'+'tic'='ACETIC'
(Other definitions for acetic that I've seen before include "Colourless pungent acid" , "sort of acid" , "Tasting like vinegar" , "Saucy'" , "really sour" .)