Ball game in uniform aboard ship (6)
I believe the answer is:
sevens
'ball game' is the definition.
'uniform aboard ship' is the wordplay.
'uniform' becomes 'even' (I've seen this before).
'aboard' means one lot of letters goes inside another.
'ship' becomes 'ss'.
'even' put into 'ss' is 'SEVENS'.
'in' acts as a link.
(Other definitions for sevens that I've seen before include "rugby competition" , "Card game; form of rugby" , "cards!" , "Form of Rugby Union" , "77" .)