Boat to pull larger ones (3)
I believe the answer is:
tug
'boat to pull larger ones' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tug that I've seen before include "Pull like small boat" , "Boat to pull larger ones" , "Pull vigorously and a boat that does it" , "draw with some effort" , "Pull hard; boat" .)