Good doctor laid into King - he's always moaning (8)

I believe the answer is:

grumbler

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'he's always moaning' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'good doctor laid into king' is the wordplay.
'good' becomes 'g' (abbreviation).
'doctor' becomes 'MB' (abbreviation for Bachelor of Medicine).
'laid into' indicates putting letters inside.
'king' becomes 'ruler' (I've seen this before).
'mb' going into 'ruler' is 'rumbler'.
'g'+'rumbler'='GRUMBLER'

Can you help me to learn more?

(Other definitions for grumbler that I've seen before include "Persistent complainer" , "One who complains" .)

I've seen this clue in The Independent.

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