Article penned by Her Majesty? It might send you to sleep (5)
I believe the answer is:
ether
'it might send you to sleep' is the definition.
'article penned by her majesty?' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'penned by' indicates putting letters inside.
'her majesty?' becomes 'er' (Elizabeth Regina).
'the' inserted inside 'er' is 'ETHER'.
(Other definitions for ether that I've seen before include "Anaesthetic; clear sky (poetic)" , "The gas is there, oddly" , "Some were put out by this" , "Colourless volatile highly inflammable liquid" , "What's clear" .)