Seats become rotten aboard ship (7)
I believe the answer is:
saddles
'seats' is the definition.
(saddle is a kind of seat)
'rotten aboard ship' is the wordplay.
'rotten' becomes 'addle' (I've seen this before).
'aboard' means one lot of letters goes inside another.
'ship' becomes 'ss'.
'addle' put into 'ss' is 'SADDLES'.
'become' acts as a link.
(Other definitions for saddles that I've seen before include "Burdens" , "Seats for horses" , "Riding seats" , "These joints of meat" , "Jockeys' seats" .)