Support for illumination display under 50A? (8)
I believe the answer is:
lamppost
'support for illumination' is the definition.
(a lamppost is a source of illumination)
'display under 50a?' is the wordplay.
'display' becomes 'post' (I've seen this in other clues).
'under' means one lot of letters go next to another (below, in a down clue).
'50a?' becomes 'lamp' (I can't explain this - if you can you should give a lot more credence to this answer).
'post' after 'lamp' is 'LAMPPOST'.
(Other definitions for lamppost that I've seen before include "Enlightenment can be obtained from this" , "Street furniture on which George Formby would lean in song" , "Pole, light-headed" , "Street light supporter" , "Street light (on which George Formby leant?)" .)