Suppose one includes a mathematical symbol (5)

I believe the answer is:

opine

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'suppose' is the definition.
(I know that suppose can be written as opine)

'one includes a mathematical symbol' is the wordplay.
'includes' indicates putting letters inside.
'a mathematical symbol' becomes 'pi' (pi is a kind of symbol. I am not sure about the 'mathematical' bit.).
'one' enclosing 'pi' is 'OPINE'.

Can you help me to learn more?

(Other definitions for opine that I've seen before include "Conjecture" , "Suppose" , "State one's held view" , "State as one's belief" , "State as one's opinion" .)

I've seen this clue in The Globe and Mail.

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