Significant number held up party game ... (7)
I believe the answer is:
netball
'game' is the definition.
(netball is a kind of game)
'significant number held up party' is the wordplay.
'significant number' becomes 'ten' (eg a large group of people).
'held up' says the letters should be written backwards.
'party' becomes 'ball' (ball is a kind of party).
'ten' reversed gives 'net'.
'net'+'ball'='NETBALL'
(Other definitions for netball that I've seen before include "Team sport" , "Sport whose players wear bibs" , "Team game/sport" , "Team game for tall Ben" , "activity no longer associated solely with girls" .)