Delightful new diamonds (4)
I believe the answer is:
nice
'delightful' is the definition.
(thesaurus)
'new diamonds' is the wordplay.
'new' becomes 'n'.
'diamonds' becomes 'ice' (I've seen this before).
'n'+'ice'='NICE'
(Other definitions for nice that I've seen before include "Fine as in distinction, or pleasant" , "French city - sounds pleasant" , "Pleasant, agreeable" , "A pleasant French city? ( 4)" , "Agreeable -- subtle" .)