Extra-large collars no good for cur (7)
I believe the answer is:
mongrel
'for cur' is the definition.
(synonyms)
'extra-large collars no good' is the wordplay.
'extra' becomes 'more' (both can mean additional).
'large' becomes 'L' (eg in clothes sizes).
'collars' indicates putting letters inside (to collar can mean to apprehend or capture).
'no good' becomes 'ng' (abbreviation).
'more'+'l'='morel'
'morel' placed around 'ng' is 'MONGREL'.
(Other definitions for mongrel that I've seen before include "Dog of no recognisable breed" , "Mixed-breed dog" , "bastard" , "unlike well-bred setter, say" , "Cross-breed dog" .)