Thought you'd swallowed it (8)

I believe the answer is:

believed

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'thought' is the definition.
('believe' can be a synonym of 'think')

I cannot understand how the remainder of the clue works.

(Other definitions for believed that I've seen before include "Credited" , "Regarded as true" , "Accepted as true" , "Thought" .)

I've seen this clue in the Evening Standard.

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