Form of pig pen in the forest (6)
I believe the answer is:
epping
'forest' is the definition.
(I've seen this before)
'form of pig pen' is the wordplay.
'form of' is an anagram indicator (the letters need to be in another 'form').
'pig'+'pen'='pigpen'
'pigpen' anagrammed gives 'EPPING'.
'in the' is the link.
(Other definitions for epping that I've seen before include "Forest NE of London" , "Essex site of a forest" , "Forest near London" .)