White House finally stops copper delivery (3,4)
I believe the answer is:
cue ball
'white' is the definition.
(the cue ball in snooker is also called the white)
'house finally stops copper delivery' is the wordplay.
'finally' suggests the final letters.
'stops' means one lot of letters goes inside another (stopping can mean blocking a gap).
'copper' becomes 'cu' (chemical symbol).
'delivery' becomes 'ball' (ball is a kind of delivery).
The final letter of 'house' is 'e'.
'cu'+'ball'='cuball'
'e' inserted inside 'cuball' is 'CUE-BALL'.
(Other definitions for cue ball that I've seen before include "Jimmy White's white?" , "Object struck by a snooker player" , "The white in snooker" , "Feature of pub entertainment" .)