Boat fit to be used in war, gutted (6)
I believe the answer is:
whaler
'boat' is the definition.
(thesaurus)
'fit to be used in war gutted' is the wordplay.
'fit' becomes 'hale' (both can mean healthy).
'to be used in' is an insertion indicator.
'gutted' means to remove the middle letters (to gut something is to remove its insides).
'war' with its middle removed is 'wr'.
'hale' placed into 'wr' is 'WHALER'.
(Other definitions for whaler that I've seen before include "Occupation of Melville's Captain Ahab" , "Aussie tramp?" , "The Pequod?" , "Ship armed with harpoons" , "Mammal-hunting boat" .)