Hermit, say, is sour about Society (7)
I believe the answer is:
ascetic
'hermit' is the definition.
(I've seen this before)
'is sour about society' is the wordplay.
'is sour' becomes 'acetic' (I've seen this before).
'about' indicates putting letters inside.
'society' becomes 's' (common abbreviation in names of organisations).
'acetic' placed around 's' is 'ASCETIC'.
'say' acts as a link.
(Other definitions for ascetic that I've seen before include "One abstaining from worldly comforts" , "Self-denying, often for religious reasons" , "Self-denying, Spartan" , "Hermit" , "Austere disciple" .)