Child joining a large comprehensive (5)
I believe the answer is:
total
'comprehensive' is the definition.
(I know that comprehensive can be written as total)
'child joining a large' is the wordplay.
'child' becomes 'tot' (term for a young child).
'joining' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'tot'+'a'+'l'='TOTAL'
(Other definitions for total that I've seen before include "final answer" , "Full, entire amount" , "Sum, aggregation" , "wreck" , "given by 8 [ADDING MACHINES]" .)