Type of warship expected to conquer (6)
I believe the answer is:
subdue
'to conquer' is the definition.
('subdue' can be a synonym of 'conquer')
'type of warship expected' is the wordplay.
'type of warship' becomes 'sub'.
'expected' becomes 'due' (similar in meaning).
'sub'+'due'='SUBDUE'
(Other definitions for subdue that I've seen before include "Vanquish" , "'Quieten, bring under control (6)'" , "Conquer" , "Moderate" , "Overcome, force to yield" .)