What someone on rooftop must do? Become depressed (3,4)
I believe the answer is:
get down
'what someone on rooftop must do?' is the definition.
(get down from the roof)
'become depressed' is the wordplay.
'become' becomes 'get' (synonyms).
'depressed' becomes 'down' (synonyms).
'get'+'down'='GET DOWN'
(Other definitions for get down that I've seen before include "Dismount, alight" , "See feathers, > alight" , "you could - from duck" , "Swallow" .)