Rig in what must be warship (7)
I believe the answer is:
frigate
'warship' is the definition.
(I know that frigate is a type of warship)
'rig in what must be' is the wordplay.
'in' means one lot of letters goes inside another.
'what must be' becomes 'fate' (I've seen this before).
'rig' going inside 'fate' is 'FRIGATE'.
(Other definitions for frigate that I've seen before include "Type of warship" , "Naval escort vessel" , "US warship" , "Ship" , "Warship - seabird" .)