Illuminated concerning unit (5)
I believe the answer is:
litre
'unit' is the definition.
(litre is a kind of unit)
'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 "less than two pints" , "It's equal to about 1.75 pints" , "One cubic decimetre" , "Fluid measure" , "Metric liquid measure" .)