Hero gets cold during sea travel (8)
I believe the answer is:
achilles
'hero' is the definition.
(I've seen this before)
'cold during sea travel' is the wordplay.
'cold' becomes 'chill' (chill can mean cold).
'during' means one lot of letters goes inside another.
'travel' is an anagram indicator (I've seen 'travelling' mean this).
'sea' anagrammed gives 'aes'.
'chill' placed inside 'aes' is 'ACHILLES'.
'gets' acts as a link.
(Other definitions for achilles that I've seen before include "Legendary Greek with injured heel" , "Cause of Hector's death" , "Greek hero with a weak point" , "Homeric hero" , "man with a particular weakness" .)