Track ship mostly (4)
I believe the answer is:
line
'track' is the definition.
(line is a kind of track)
'ship mostly' is the wordplay.
'ship' becomes 'liner' (liner is a kind of ship).
'mostly' means to remove the last letter (most but not all of the word).
'liner' with its final letter taken off is 'LINE'.
(Other definitions for line that I've seen before include "(Skin) wrinkle; limit" , "A length without breadth or thickness" , "Row - occupation" , "Wrinkle; row" , "short note" .)