Illuminated regarding measure (5)
I believe the answer is:
litre
'measure' is the definition.
(litre is a kind of measure)
'illuminated regarding' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'regarding' becomes 're' (re can mean regarding, concerning).
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "non-imperial measure" , "One cubic decimetre" , "Metric unit of volume" , "Fluid measure" , "less than two pints" .)