Warship in fix, beset by doom (7)
I believe the answer is:
frigate
'warship' is the definition.
(I know that frigate is a type of warship)
'fix beset by doom' is the wordplay.
'fix' becomes 'rig' (rig can mean to fix or manipulate).
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'doom' becomes 'fate' (synonyms).
'rig' going within 'fate' is 'FRIGATE'.
'in' is the link.
(Other definitions for frigate that I've seen before include "Light warship" , "Type of warship" , "Warship; -- bird" , "Ship" , "Naval escort vessel" .)