It's a wobbly rooftop step (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
'it's a wobbly rooftop' is the wordplay.
'wobbly' indicates anagramming the letters.
'rooftop' becomes 'r' (top of the word 'roof').
'its'+'a'='itsa'
'itsa' with letters rearranged gives 'stai'.
'stai'+'r'='STAIR'
(Other definitions for stair that I've seen before include "Set of steps" , "means to go down" , "section of flight" , "Bit of a flight!" , "Component of flight" .)