Small boat in rough (6)
I believe the answer is:
sketch
'rough' is the definition.
(thesaurus)
'small boat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'boat' becomes 'ketch' (ketch is a type of sailing boat).
's'+'ketch'='SKETCH'
'in' is the link.
(Other definitions for sketch that I've seen before include "Rough outline" , "Short account" , "Small sailing boat" , "Simple drawing" , "Basic drawing" .)