Free to include new skin (4)
I believe the answer is:
rind
'skin' is the definition.
('rind' can be a synonym of 'skin')
'free to include new' is the wordplay.
'free' becomes 'rid' ('rid' can be a synonym of 'free'**).
'to include' is an insertion indicator.
'new' becomes 'n'.
'rid' going around 'n' is 'RIND'.
(Other definitions for rind that I've seen before include "Tough outer layer" , "'Peel, bark (4)'" , "Skin of orange or melon" , "Protective cover on cheese" , "Crust" .)