Unhappy about new beach (4)
I believe the answer is:
sand
'beach' is the definition.
'unhappy about new' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy'**).
'about' is an insertion indicator.
'new' becomes 'n'.
'sad' enclosing 'n' is 'SAND'.
(Other definitions for sand that I've seen before include "It's used in glass-making" , "Beach/desert material" , "Smooth down (surface)" , "Grit; powdered rock" , "It's used to make glass" .)