Illuminated about amount (5)
I believe the answer is:
litre
'amount' is the definition.
(litre is a kind of amount)
'illuminated about' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'about' becomes 're' (regarding).
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "Mcc?" , "One thousand cubic centimetres" , "It's equal to about 1.75 pints" , "non-imperial measure" , "Metric unit of volume" .)