Seen event toppling figure (9)
I believe the answer is:
seventeen
'figure' is the definition.
(I've seen this before)
'seen event toppling' is the wordplay.
'toppling' is an anagram indicator (I've seen 'toppled' mean this (the order of letters topples or collapses)).
'seen'+'event'='seenevent'
'seenevent' anagrammed gives 'SEVENTEEN'.
(Other definitions for seventeen that I've seen before include "Cardinal number" , "Figure" , "next clue" , "Quite a few" , "Age when full driving licence can be obtained" .)