Damage on outside of round large tooth (5)
I believe the answer is:
molar
'tooth' is the definition.
(molar is a kind of tooth)
'damage on outside of round large' is the wordplay.
'damage' becomes 'mar' (marring is a kind of damaging).
'on outside of' indicates putting letters inside.
'round' becomes 'o' (round can mean a round object).
'large' becomes 'l'.
'o'+'l'='ol'
'mar' placed around 'ol' is 'MOLAR'.
(Other definitions for molar that I've seen before include "Broad-crowned tooth" , "Dealing with masses" , "part of the daily grind" , "Large tooth" , "that could be capped" .)