The girl with new hat gets a cocktail dress (6)
I believe the answer is:
sheath
'a cocktail dress' is the definition.
('sheath' can be a synonym of 'cocktail dress')
'the girl with new hat' is the wordplay.
'the girl' becomes 'she' (**).
'with' says to put letters next to each other.
'new' indicates an anagram.
'hat' is an anagram of 'ath'.
'she'+'ath'='SHEATH'
'gets' is the link.
(Other definitions for sheath that I've seen before include "Close fitting dress" , "Blade-cover" , "Cover protecting" , "Condom" , "Sword covering" .)