A tree makes a girl depressed (6)
I believe the answer is:
sallow
'a tree' is the definition.
(sallow is a kind of tree)
'a girl depressed' is the wordplay.
'a girl' becomes 'sal' (feminine given name).
'depressed' becomes 'low' (synonyms).
'sal'+'low'='SALLOW'
'makes' is the link.
(Other definitions for sallow that I've seen before include "Unhealthy looking" , "Sickly complexion" , "of a pale yellow or brown colour" , "Of skin, yellowish in colour" , "Pale; sickly" .)