Underwater vessel expected to overcome? (6)
I believe the answer is:
subdue
'overcome?' is the definition.
(subduing is a kind of overcoming)
'underwater vessel expected' is the wordplay.
'underwater vessel' becomes 'sub' (submarine).
'expected' becomes 'due' (similar in meaning).
'sub'+'due'='SUBDUE'
'to' is the link.
(Other definitions for subdue that I've seen before include "Bring under control, tame" , "Moderate" , "Put down - quieten" , "Quell" , "Overcome or inhibit" .)