A tower of strength (3)
I believe the answer is:
tug
'a tower' is the definition.
(something that tows)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tug that I've seen before include "Pull hard or suddenly" , "Pull at the towing vessel" , "Towboat" , "Pull a form of boat" , "Working boat" .)