Illuminated concerning quantity (5)
I believe the answer is:
litre
'quantity' is the definition.
(litre is a kind of quantity)
'illuminated concerning' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'concerning' becomes 're' (both can mean on a particular topic).
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "quantity of drink" , "Approx 1 point 75 pints" , "A pint or two" , "One thousand cc" , "Amount of liquid" .)