False belief held by Ptolemy, then (4)

I believe the answer is:

myth

'false belief' is the definition.
(I've seen this before)

'held by ptolemy then' is the wordplay.
'held by' indicates the answer is hidden within the clue.
'MYTH' can be found hidden inside 'ptolemy then'.

(Other definitions for myth that I've seen before include "There's no truth in it" , "Traditional story not based on fact" , "Fictitious thing; traditional story" , "Legend; fable" , "A US president; a UK PM" .)

I've seen this clue in The Telegraph.

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