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