Illuminated about quantity (5)
I believe the answer is:
litre
'quantity' is the definition.
(litre is a kind of quantity)
'illuminated about' is the wordplay.
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'about' becomes 're' (regarding).
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "less than two pints" , "quantity of drink" , "It's equal to about 1.75 pints" , "Unit of liquid measure" , "Measure of liquid volume" .)