Guess pet clause was used (9)

I believe the answer is:

speculate

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'guess' is the definition.
(I know that guess is a kind of speculate)

'pet clause was used' is the wordplay.
'pet clause' can be anagrammed to 'SPECULATE'.
Though, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for speculate that I've seen before include "Consider" , "Theorise without firm evidence" , "Buy in hope of profit, but with risk of loss" , "As before, observe" , "Gamble on investments" .)

I've seen this clue in The Mirror.

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