Pains grip sick Greek hero (8)
I believe the answer is:
achilles
'greek hero' is the definition.
(I've seen this before)
'pains grip sick' is the wordplay.
'pains' becomes 'aches' (ache is a kind of pain).
'grip' is an insertion indicator.
'sick' becomes 'ill' (similar in meaning).
'aches' placed around 'ill' is 'ACHILLES'.
(Other definitions for achilles that I've seen before include "Ancient Greek with weak heel" , "Famous Greek" , "Greek hero who gave his name to a bad heel" , "Heroic warrior" , "Warrior of old" .)