Large stain ruined old pegs (7)
I believe the answer is:
splodge
'large stain' is the definition.
(a splodge is a stain)
'ruined old pegs' is the wordplay.
'ruined' indicates anagramming the letters.
'old'+'pegs'='oldpegs'
'oldpegs' anagrammed gives 'SPLODGE'.
(Other definitions for splodge that I've seen before include "Irregular blot" , "Dirty mark" , "Messy spot" , "Spot, splash, smear" , "big splash" .)