Child attending comprehensive gets good mark for addition (9)
I believe the answer is:
totalling
'addition' is the definition.
(I've seen this before)
'child attending comprehensive gets good' is the wordplay.
'child' becomes 'tot' (I've seen this before).
'attending' is a charade indicator (letters next to each other).
'comprehensive' becomes 'allin' (I've seen this before).
'gets' is a charade indicator (letters next to each other).
'good' becomes 'g' (abbreviation).
'tot'+'allin'+'g'='TOTALLING'
'mark for' is the link.
(Other definitions for totalling that I've seen before include "Coming to" , "Summing to" , "Addition" .)