Seaman in boat a ruffian (3)
I believe the answer is:
tar
'seaman' is the definition.
(Jack Tar is an informal term for a sailor)
'in boat a ruffian' is the wordplay.
'in' indicates a hidden word.
'TAR' can be found hidden inside 'boat a ruffian'.
(Other definitions for tar that I've seen before include "It may cover the road" , "Slang sailor or road surfacing material" , "Sticky substance used in road construction" , "used by those who pay to work on deck" , "prepare for feathers?" .)