Young child attending a large comprehensive (5)
I believe the answer is:
total
'comprehensive' is the definition.
(thesaurus)
'young child attending a large' is the wordplay.
'young child' becomes 'tot' (I've seen this before).
'attending' says to put letters next to each other.
'large' becomes 'l'.
'tot'+'a'+'l'='TOTAL'
(Other definitions for total that I've seen before include "Whole, complete" , "outright" , "Come to" , "Final tally" , "Including all" .)