Sailor in boat a rascal (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'in boat a rascal' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TAR' can be found hidden inside 'boat a rascal'.
(Other definitions for tar that I've seen before include "Cigarette substance" , "prepare for feathers?" , "Viscous liquid; sailor" , "Familiar sailor - what a rat" , "Viscous bituminous substance" .)