Tom turns up on a ship (4)
I believe the answer is:
puss
'tom' is the definition.
(thesaurus)
'turns up on a ship' is the wordplay.
'turns' says the letters should be written backwards.
'up on' becomes 'up' ('up' can be similar in meaning to 'up on').
'a ship' becomes 'ss'.
'up' back-to-front is 'pu'.
'pu'+'ss'='PUSS'
(Other definitions for puss that I've seen before include "Pet" , "Booted panto character" , "Moggie" , "Cat (abbr)" , "It might be a tortoiseshell" .)