Window supporter of little substance (8)
I believe the answer is:
fanlight
'window' is the definition.
(I know that fanlight is a type of window)
'supporter of little substance' is the wordplay.
'supporter' becomes 'fan' (fan is a kind of supporter).
'of little substance' becomes 'light' (I can't justify this - if you can you should believe this answer much more).
'fan'+'light'='FANLIGHT'
Can you help me to learn more?
(Other definitions for fanlight that I've seen before include "Small window" , "It enables us to see" , "opening over the door" , "elevated aperture" , "Windows above a door" .)