One critical of Her Majesty going on strike (7)
I believe the answer is:
knocker
'one critical' is the definition.
(someone who knocks or criticises things)
'her majesty going on strike' is the wordplay.
'her majesty' becomes 'ER' (Elizabeth Regina).
'going on' says to put letters next to each other.
'strike' becomes 'knock' (knocking is a kind of striking).
'er' put after 'knock' is 'KNOCKER'.
'of' is the link.
(Other definitions for knocker that I've seen before include "Door banger -- detractor" , "Hinged device on a door" , "One strongly criticising" , "Alternative to door bell" , "Something on door; one continually finding fault" .)