Queen rings the number (5)
I believe the answer is:
ether
'number' is the definition.
(I've seen this before)
'queen rings the' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'rings' indicates putting letters inside.
'er' going around 'the' is 'ETHER'.
(Other definitions for ether that I've seen before include "Light liquid used as anaesthetic" , "The gas is there, oddly" , "Some were put out by this" , "Colourless volatile highly inflammable liquid" , "Rare atmosphere" .)