Article inspired by Her Majesty’s knockout drops? (5)
I believe the answer is:
ether
'knockout drops?' is the definition.
(ether is a type of anaesthetic)
'article inspired by her majesty's' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'inspired by' means one lot of letters goes inside another.
'her majesty' becomes 'er' (Elizabeth Regina).
'the' put inside 'er' is 'ETHER'.
(Other definitions for ether that I've seen before include "Colourless volatile highly inflammable liquid" , "Anaesthetic liquid; clear sky" , "Rare atmosphere" , "Type of chemical" , "Volatile organic liquid once used as a general anaesthetic" .)