Sailor in boat a rating (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'in boat a rating' is the wordplay.
'in' indicates a hidden word.
'TAR' can be found hidden inside 'boat a rating'.
(Other definitions for tar that I've seen before include "Art (anag.)" , "Kind of rat for sailor" , "used by those who pay to work on deck" , "It paves the way" , "not put on ship in proverbial case of false economy?" .)