Sailor during offensive is likely to survive (6)
I believe the answer is:
viable
'likely to survive' is the definition.
'sailor during offensive' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'during' indicates putting letters inside.
'offensive' becomes 'vile' (similar in meaning).
'ab' put into 'vile' is 'VIABLE'.
'is' is the link.
(Other definitions for viable that I've seen before include "having potential to succeed" , "Practicable, able to be done successfully" , "Capable of life and growth" , "Capable of living" , "Having a prospect of success" .)