Child beat legend coming to (9)
I believe the answer is:
totalling
'coming to' is the definition.
(I've seen this before)
'child beat legend' is the wordplay.
'child' becomes 'tot' (term for a young child).
'beat' becomes 'allin' (synonyms).
'legend' becomes 'g' (leg-end - end letter of 'leg').
'tot'+'allin'+'g'='TOTALLING'
(Other definitions for totalling that I've seen before include "Summing to" , "Addition" , "Coming to" .)