Having thoughts about spare ruler (8)

I believe the answer is:

thinking

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'having thoughts' is the definition.
I can't judge whether this defines the answer.

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

'about' is the link.

Can you help me to learn more?

(Other definitions for thinking that I've seen before include "giving opinion" , "Pondering" , "Fancying" , "Thought" , "rationale" .)

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

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