Tart giving saucier twitch, occasionally (6)
I believe the answer is:
acetic
'tart' is the definition.
(I've seen this before)
'saucier twitch occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'saucier'+'twitch'='sauciertwitch'
The alternate letters of 'sauciertwitch' are 'ACETIC'.
'giving' acts as a link.
(Other definitions for acetic that I've seen before include "in relation to acid" , "Like vinegar's acid" , "Like the acid in vinegar" , "The acid in vingear" , "Vinegar-like" .)