Accepts birds are raised on board a ship (7)
I believe the answer is:
assumes
'accepts' is the definition.
('assume' can be a synonym of 'accept')
'birds are raised on board a ship' is the wordplay.
'birds' becomes 'emus' (emu is a kind of bird**).
'are raised' says the letters should be written in reverse.
'on board' means one lot of letters goes inside another.
'ship' becomes 'ss'.
'emus' reversed gives 'sume'.
'a'+'ss'='ass'
'sume' going into 'ass' is 'ASSUMES'.
(Other definitions for assumes that I've seen before include "dons" , "Takes to be the case" , "Take to be the case" , "Takes for granted or puts on" , "Puts on; accepts" .)