Boomer king of the castle (4)

I believe the answer is:

rook

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This might be a double definition.

'boomer king' is the first definition.
'rook' can be an answer for 'king' (thesaurus). I am unsure of the 'boomer' bit.

'the castle' is the second definition.
('rook' can be a synonym of 'castle')

'of' acts as a link.

Can you help me to learn more?

(Other definitions for rook that I've seen before include "Defraud -- crow" , "Use deceit to get money" , "Black bird - cheat - chess piece" , "Rank - and - file man" , "journalist Jean" .)

I've seen this clue in the Sydney Morning Herald.

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