Warship expected to conquer (6)
I believe the answer is:
subdue
'to conquer' is the definition.
(synonyms)
'warship expected' is the wordplay.
'warship' becomes 'sub' (short for submarine, a submersible warship).
'expected' becomes 'due' (similar in meaning).
'sub'+'due'='SUBDUE'
(Other definitions for subdue that I've seen before include "Vanquish" , "Quash" , "Quell" , "Put down - quieten" , "Force to yield, tame" .)