North Rocks Fair (4)
I believe the answer is:
nice
'fair' is the definition.
(thesaurus)
'north rocks' is the wordplay.
'north' becomes 'N' (abbreviation).
'rocks' becomes 'ice' (both can informally mean diamonds).
'n'+'ice'='NICE'
(Other definitions for nice that I've seen before include "Charming" , "French Mediterranean resort" , "Pleasant-sounding city in S France" , "Pleasant - French resort" , "French city - sounds plesant" .)