What's up? (3)
(Other definitions for doc that I've seen before include "Consultant" , "Medic (familiar)" , "Medic, in short" , "Holliday (in 17 24)" , "Quack" .)
'what's up?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sky that I've seen before include "Upper region, empyrean" , "Hit high into the air" , "Weather" , "-- Blue" , "Hit (a ball) very high" .)
(Other definitions for roof that I've seen before include "See 24" , "Top covering of a building or vehicle" , "Upper limit; house top" , "Covering of house" , "Cover for house" .)