Sailors not out in boat should be going off (7)
I believe the answer is:
turning
'going off' is the definition.
(eg rotting food turns or goes off)
'sailors not out in boat' is the wordplay.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'not out' becomes 'in' ('in' is the opposite to 'out').
'in' is an insertion indicator.
'boat' becomes 'tug' (tug is a kind of boat**).
'rn'+'in'='rnin'
'rnin' put within 'tug' is 'TURNING'.
'should be' is the link.
(Other definitions for turning that I've seen before include "Rotating; going bad" , "Changing allegiance" , "Rotating; road off another" , "Rotating; junction" , "Deviation" .)