Depressed? Do cocaine before farewell (7)
I believe the answer is:
concave
'depressed?' is the definition.
('concave' can be similar in meaning to 'depressed')
'do cocaine before farewell' is the wordplay.
'do' becomes 'con'.
'cocaine' becomes 'c' (drugs abbreviation).
'before' says to put letters next to each other.
'farewell' becomes 'ave' (Latin salutation).
'con'+'c'+'ave'='CONCAVE'
(Other definitions for concave that I've seen before include "Bowing" , "Arch" , "Curving inward" , "Curved inwards" , "Curving inwards" .)