Zealous Republican in advanced depression (6)
I believe the answer is:
ardent
'zealous' is the definition.
(synonyms)
'republican in advanced depression' is the wordplay.
'republican' becomes 'R' (abbreviation).
'in' is an insertion indicator.
'advanced' becomes 'a' (abbreviation).
'depression' becomes 'dent' (dent is a kind of depression).
'a'+'dent'='adent'
'r' placed within 'adent' is 'ARDENT'.
(Other definitions for ardent that I've seen before include "Zealous - ranted (anag)" , "Fiery and zealous" , "Enthusiastic, eager" , "Hot" , "Very keen" .)