Naval vessel needing fortune to fix inside (7)
I believe the answer is:
frigate
'naval vessel' is the definition.
(I've seen this before)
'fortune to fix inside' is the wordplay.
'fortune' becomes 'fate' ('fate' can be a synonym of 'fortune').
'to fix' becomes 'rig' (rigging is a kind of fixing).
'inside' is an insertion indicator.
'fate' placed around 'rig' is 'FRIGATE'.
'needing' is the link.
(Other definitions for frigate that I've seen before include "Naval escort vessel" , "Warship; -- bird" , "Type of warship" , "Ship" , "US warship" .)