What can open party or ball (8)
I believe the answer is:
doorknob
'what can open' is the definition.
(doorknobs open doors)
'party or ball' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'ball' becomes 'knob' (I've seen this before).
'do'+'or'+'knob'='DOORKNOB'
(Other definitions for doorknob that I've seen before include "It may be turned to gain entrance" , "Turn it to gain entry" , "Handle for portal" , "Round handle" , "Latch opener" .)