Boatman's transport's popular (4)
I believe the answer is:
ship
'boatman's transport's' is the definition.
'ship' can be an answer for 'transport's' (ship is a kind of transport). I am not certain of the 'boatman's' bit.
'popular' is the wordplay.
I cannot really see how this works, but
'popular' could be 'hip' (hip can mean trendy or popular) and 'hip' is found within the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ship that I've seen before include "craft of 1d [ADVENTURE]" , "Take on" , "Sea-going craft" , "Ocean vessel" , "Navigable vessel" .)