Numbers still to be brought aboard ship (6)
I believe the answer is:
sevens
'numbers' is the definition.
(seven is a kind of number)
'still to be brought aboard ship' is the wordplay.
'still' becomes 'even' (synonyms).
'to be brought aboard' means one lot of letters goes inside another (inserted letters come inside or on board).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'even' inserted inside 'ss' is 'SEVENS'.
(Other definitions for sevens that I've seen before include "77" , "Numbers" , "rugby competition" , "Card game; form of rugby" , "Form of Rugby Union" .)