Heart of anthems in Queen number (5)
I believe the answer is:
ether
'number' is the definition.
(anaesthetic, something which numbs)
'heart of anthems in queen' is the wordplay.
'heart of' indicates the central letters.
'in' is an insertion indicator.
'queen' becomes 'ER' (short for Elizabeth Regina ie Queen Elizabeth).
The middle letters of 'anthems' are 'the'.
'the' put into 'er' is 'ETHER'.
(Other definitions for ether that I've seen before include "number" , "Some were put out by this" , "Volatile organic liquid once used as a general anaesthetic" , "Former anaesthetic" , "Chemical solvent" .)