Measure damage to syntax (7)
I believe the answer is:
grammar
'syntax' is the definition.
(I've seen this before)
'measure damage' is the wordplay.
'measure' becomes 'gram' (gram is a kind of measure).
'damage' becomes 'mar' (marring is a kind of damaging).
'gram'+'mar'='GRAMMAR'
'to' acts as a link.
(Other definitions for grammar that I've seen before include "A language's syntax and morphology" , "Rules of syntax" , "Functions and relations of words in sentences" , "Study of words and sentences" , "Syntax etc" .)