Greek hero cool in rising water (8)
I believe the answer is:
achilles
'greek hero' is the definition.
(I've seen this before)
'cool in rising water' is the wordplay.
'cool' becomes 'chill' (similar in meaning).
'in' means one lot of letters goes inside another.
'rising' shows that the letters should be reversed in order (in a down clue, letters go up).
'water' becomes 'sea' (the sea is water).
'sea' back-to-front is 'aes'.
'chill' put within 'aes' is 'ACHILLES'.
(Other definitions for achilles that I've seen before include "Legendary Greek with injured heel" , "Great Greek with weak heel" , "Homeric hero" , "His heel was his weak point" , "Greek hero with a weak point" .)