Ship could be a drifter (5)
I believe the answer is:
tramp
'a drifter' is the definition.
(I know that tramp is a type of drifter)
'ship could be' is the wordplay.
I cannot quite understand how this works, but
'be' could be 'am' and 'am' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tramp that I've seen before include "man down on his luck" , "Ship, possibly" , "Hike" , "Heavily plod around" , "Walk heavily or wearily" .)