Boxer possibly boxing on ship (9)
I believe the answer is:
freighter
'ship' is the definition.
(freighter is a kind of ship)
'boxer possibly boxing on' is the wordplay.
'boxer possibly' becomes 'fighter' (boxer is a kind of fighter).
'boxing' indicates putting letters inside ('box' can mean contain, as in to put in a box).
'on' becomes 're' (regarding).
'fighter' placed around 're' is 'FREIGHTER'.
(Other definitions for freighter that I've seen before include "Cargo-ship" , "Cargo-carrying ship or plane" , "A plane carrying large load" , "transport vessel" , "Cargo ship" .)