Considering slim monarch (8)

I believe the answer is:

thinking

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'considering' is the definition.
The answer and definition can be both related to thinking as well as being present participle verbs.
Maybe there's a link between them I don't understand?

'slim monarch' is the wordplay.
'slim' becomes 'thin' (similar in meaning).
'monarch' becomes 'king' (king is a kind of monarch).
'thin'+'king'='THINKING'

Can you help me to learn more?

(Other definitions for thinking that I've seen before include "Pondering" , "Considering in the mind" , "Thought" , "Reasoning in the mind" , "Take" .)

I've seen this clue in The Daily Express.

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