Sour habitual response by expert (6)
I believe the answer is:
acetic
'sour' is the definition.
(I've seen this before)
'habitual response by expert' is the wordplay.
'habitual response' becomes 'tic' (I've seen this in other clues).
'by' says to put letters next to each other.
'expert' becomes 'ace' (ace is a kind of expert).
'tic' put after 'ace' is 'ACETIC'.
(Other definitions for acetic that I've seen before include "Tasting like vinegar" , "Colourless pungent acid" , "Vinegary acid" , "really sour" , "sort of acid" .)