A red swelling initially hurts (5)
I believe the answer is:
aches
'hurts' is the definition.
('ache' can be a synonym of 'hurt')
'a red swelling initially' is the wordplay.
'red' becomes 'che' (Che Guevara the Marxist revolutionary).
'initially' suggests taking the first letters.
The first letter of 'swelling' is 's'.
'a'+'che'+'s'='ACHES'
(Other definitions for aches that I've seen before include "discomfort" , "Dull prolonged pains" , "Constant pains" , "Continuous pains" , "Dull persistent pains" .)