Volume was illuminating, with accompanying note (5)
I believe the answer is:
litre
'volume' is the definition.
(I've seen this before)
'was illuminating with accompanying note' is the wordplay.
'was illuminating' becomes 'lit' (I've seen this before).
'with accompanying' is a charade indicator (letters next to each other).
'note' becomes 're'.
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "quantity of drink" , "Measure of liquid volume" , "A pint or two" , "Mcc?" , "Approx 1 point 75 pints" .)