A red pine (4)
I believe the answer is:
ache
'pine' is the definition.
(synonyms)
'a red' is the wordplay.
'red' becomes 'Che' (Che Guevara the Marxist revolutionary).
'a'+'che'='ACHE'
(Other definitions for ache that I've seen before include "suffering" , "burn" , "agony" , "Longing" , "Dull continuous pain" .)