Heavens, it's not heavy, though it has made a hole in the roof (8)
I believe the answer is:
skylight
'roof' is the definition.
(thesaurus)
'heavens it's not heavy' is the wordplay.
'heavens' becomes 'sky' (sky is a kind of heavens).
'it's not heavy' becomes 'light' ('light' is the opposite to 'heavy').
'sky'+'light'='SKYLIGHT'
'though it has made a hole in the' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for skylight that I've seen before include "Window in a roof or ceiling" , "Attic window" .)