Child exhausted by midnight coming to (9)
I believe the answer is:
totalling
'coming to' is the definition.
(I've seen this before)
'child exhausted by midnight' is the wordplay.
'child' becomes 'tot' (term for a young child).
'exhausted' becomes 'all in' ('all in' can mean exhausted).
'by' says to put letters next to each other.
'midnight' becomes 'g' (the middle letter in 'night').
'tot'+'allin'+'g'='TOTALLING'
(Other definitions for totalling that I've seen before include "Coming to" , "Summing to" , "Addition" .)