Decline America's cup (7)
I believe the answer is:
atrophy
'decline' is the definition.
(I've seen this before)
'america's cup' is the wordplay.
'america' becomes 'a' (common abbreviation eg in USA).
'cup' becomes 'trophy' (I've seen this in another clue).
'a'+'trophy'='ATROPHY'
(Other definitions for atrophy that I've seen before include "Become weak and die away through lack of use" , "Waste away, wither" , "Decay" , "Waste away through under-nourishment or lack of use" , "Waste away (from disuse?)" .)