Low rafter supports new illumination (8)
I believe the answer is:
moonbeam
'illumination' is the definition.
(I have seen 'reflective illumination' mean 'moonbeam' so perhaps 'illumination' could also mean 'moonbeam')
'low rafter supports new' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'rafter' becomes 'beam' (I can't justify this - if you can you should believe this answer much more).
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'beam' after 'n' is 'nbeam'.
'moo'+'nbeam'='MOONBEAM'
Can you help me to learn more?
(Other definitions for moonbeam that I've seen before include "Night light?" , "Night shiner" , "Ray, not the brightest" , "Natural light" , "Lunar shaft of light" .)