Illuminated concerning measure (5)
I believe the answer is:
litre
'measure' is the definition.
(litre is a kind of measure)
'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 "One thousand cubic centimetres" , "Unit of capacity in metric system" , "Approx 1 point 75 pints" , "less than two pints" , "Metric unit of volume" .)