Set in middle of the forest (6)
I believe the answer is:
harden
'set' is the definition.
(I've seen this before)
'middle of the forest' is the wordplay.
'middle of' says to take the centre.
'forest' becomes 'arden' (Forest of Arden).
The centre of 'the' is 'h'.
'h'+'arden'='HARDEN'
'in' acts as a link.
(Other definitions for harden that I've seen before include "Grow more firm" , "Small fly" , "Become stronger or firmer" , "Become less soft" , "Set; become tougher" .)