Ptolemy thus embraces false notion (4)
I believe the answer is:
myth
'false notion' is the definition.
(I've seen this before)
'ptolemy thus embraces' is the wordplay.
'embraces' indicates the answer is hidden within the clue (I've seen 'embracing' mean this (the solution is embraced by words in the clue)).
'MYTH' can be found hidden inside 'ptolemy thus'.
(Other definitions for myth that I've seen before include "Ancient story" , "Delusion" , "(Urban?) story" , "false interpretation?" , "Traditional allegory" .)