Pains gripping ailing hero (8)
I believe the answer is:
achilles
'hero' is the definition.
(I've seen this before)
'pains gripping ailing' is the wordplay.
'pains' becomes 'aches' (ache is a kind of pain).
'gripping' means one lot of letters goes inside another (inserted letters are gripped).
'ailing' becomes 'ill' (similar in meaning).
'aches' enclosing 'ill' is 'ACHILLES'.
(Other definitions for achilles that I've seen before include "Mythological Greek hero" , "Greek hero of the Trojan War" , "Greek hero who gave his name to a bad heel" , "Heroic warrior" , "Warrior of old" .)