Sailor in base is capable of working (6)
I believe the answer is:
viable
'capable of working' is the definition.
(something viable is likely to work)
'sailor in base' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' means one lot of letters goes inside another.
'base' becomes 'vile' (synonyms).
'ab' going into 'vile' is 'VIABLE'.
'is' is the link.
(Other definitions for viable that I've seen before include "Feasible" , "Capable of living" , "reasonable hope of success" , "Having a chance of success" , "Able to survive; practicable" .)