Old ascetic service boss jails city revolutionary (5)
I believe the answer is:
cynic
'old ascetic' is the definition.
(type of ancient philosopher)
'service boss jails city revolutionary' is the wordplay.
'service boss' becomes 'CIC' (commander in chief, head of military services).
'jails' is an insertion indicator (inserted letters are kept or jailed).
'city' becomes 'ny' (short for New York).
'revolutionary' shows that the letters should be reversed in order.
'cic' going around 'ny' is 'cinyc'.
'cinyc' back-to-front is 'CYNIC'.
(Other definitions for cynic that I've seen before include "One who believes people's motives are always selfish" , "Misanthrope" , "Pessimistic viewer of human motives" , "One thinking the worst" , "Scoffer" .)