Back of the forearm (4)
I believe the answer is:
rear
'back' is the definition.
('rear' can be a synonym of 'back')
'of the forearm' is the wordplay.
'of' indicates a hidden word.
'REAR' can be found hidden inside 'forearm'.
(Other definitions for rear that I've seen before include "Rare at the back" , "Bring up (children)" , "Breed or bring up" , "Foster" , "Bring up and care for" .)