Watch out, monarch gets a beating (9)
I believe the answer is:
thwacking
'a beating' is the definition.
'watch out monarch' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'monarch' becomes 'king' (king is a kind of monarch).
'watch' anagrammed gives 'thwac'.
'thwac'+'king'='THWACKING'
'gets' is the link.
(Other definitions for thwacking that I've seen before include "producing a loud hit" , "striking" .)