Considering an angular 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?

'an angular monarch' is the wordplay.
'an angular' becomes 'thin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'monarch' becomes 'king' (male monarch).
'thin'+'king'='THINKING'

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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