Girl and new sweetheart (7)
I believe the answer is:
darling
'sweetheart' is the definition.
(both are terms of address to romantic partner)
'girl and new' is the wordplay.
'new' indicates anagramming the letters.
'girl'+'and'='girland'
'girland' with letters rearranged gives 'DARLING'.
(Other definitions for darling that I've seen before include "Term of endearment" , "Beloved" , "Wendy's family (Peter Pan)" , "Australian tributary of the Murray River, sweetheart" , "Favourite" .)