Illuminated concerning amount (5)
I believe the answer is:
litre
'amount' is the definition.
(litre is a kind of amount)
'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 "Nearly two pints" , "Mcc?" , "Fluid measure" , "1.7598 UK pints" , "Metric measure of capacity" .)