Girl disheartened during month in forest (6)
I believe the answer is:
jungle
'forest' is the definition.
(jungle is a kind of forest)
'girl disheartened during month' is the wordplay.
'disheartened' means to remove the middle letters.
'during' means one lot of letters goes inside another.
'month' becomes 'june' (I've seen this before**).
'girl' with its centre removed is 'gl'.
'gl' inserted within 'june' is 'JUNGLE'.
'in' is the link.
(Other definitions for jungle that I've seen before include "The __ Book (Kipling work)" , "Deep tropical forest" , "Tropical rainforest area" , "Tarzan's home" , "Dense tropical forest" .)