His Majesty quietly eating a fish (4)
I believe the answer is:
carp
'fish' is the definition.
(I know that carp is a type of fish)
'his majesty quietly eating a' is the wordplay.
'his majesty' becomes 'CR' (Charles Rex - ie King Charles).
'quietly' becomes 'p' (p for piano and quiet).
'eating' indicates putting letters inside.
'cr'+'p'='crp'
'crp' enclosing 'a' is 'CARP'.
(Other definitions for carp that I've seen before include "Fish - grouse" , "praise? The opposite" , "Cavil - fish" , "Fish often kept in ponds" , "Complain in manner of goldfish" .)