Rabbit-catcher to agonise about monarch (6)
I believe the answer is:
ferret
'rabbit-catcher' is the definition.
I can't judge whether this definition defines the answer.
'agonise about monarch' is the wordplay.
'agonise' becomes 'fret' (I've seen this in another clue).
'about' indicates putting letters inside.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'fret' enclosing 'er' is 'FERRET'.
'to' is the link.
Can you help me to learn more?
(Other definitions for ferret that I've seen before include "Rabbit-hunting polecat" , "It drives rabbits from their holes" , "Polecat bred for hunting rats and rabbits" , "Root around" , "Assiduous searcher" .)