Lightweight damage to a school (7)
I believe the answer is:
grammar
'a school' is the definition.
(grammar school is a kind of school)
'lightweight damage' is the wordplay.
'lightweight' becomes 'gram'.
'damage' becomes 'mar' (marring is a kind of damaging).
'gram'+'mar'='GRAMMAR'
'to' is the link.
(Other definitions for grammar that I've seen before include "Science of language" , "A language's syntax and morphology" , "Basic rules of language" , "Study of basic linguistic units" , "Rules of syntax" .)