Speculate, conjecture (8)

I believe the answer is:

theorise

This clue is a double definition.

'speculate' is the first definition.
(synonyms)

'conjecture' is the second definition.
(synonyms)

(Other definitions for theorise that I've seen before include "Think up possible explanations" , "Form view" , "Express ideas" , "Conjecture, hypothesise" , "Speculate by reasoning" .)

I've seen this clue in The Times and The Guardian.

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