Pains gripping sick Greek hero (8)
I believe the answer is:
achilles
'greek hero' is the definition.
(I've seen this before)
'pains gripping sick' is the wordplay.
'pains' becomes 'aches' (ache is a kind of pain).
'gripping' is an insertion indicator.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'aches' placed around 'ill' is 'ACHILLES'.
(Other definitions for achilles that I've seen before include "His heel was his weak point" , "Greek hero with vulnerable heel" , "Mythical Greek warrior with a foot problem" , "Legendary hero with weak spot" , "Greek warrior (with vulnerable heel)" .)