Tug-boat? (6)
I believe the answer is:
lugger
'boat?' is the definition.
(lugger is a kind of boat)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for lugger that I've seen before include "Small sailing vessel" , "Small ship with four-cornered sails" , "Small vessel with square sails" , "Small working boat" , "One pulling - small boat" .)