Airmen crashed in the sea (6)
I believe the answer is:
marine
'in the sea' is the definition.
'airmen crashed' is the wordplay.
'crashed' indicates an anagram.
'airmen' with letters rearranged gives 'MARINE'.
(Other definitions for marine that I've seen before include "Airmen (anag.)" , "Soldier serving on land and sea" , "Concerning the sea" , "Relating to the sea" , "'Nautical, maritime (6)'" .)