A revolutionary in pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'a revolutionary' is the wordplay.
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary**).
'a'+'che'='ACHE'
'in' is the link.
(Other definitions for ache that I've seen before include "Hurt; yearn" , "Suffer dull pain" , "Longing" , "Continuous hurt" , "Dull prolonged pain" .)