Revolutionary part of Ptolemaic astronomy (6)
I believe the answer is:
castro
'revolutionary' is the definition.
(I know this)
'part of ptolemaic astronomy' is the wordplay.
'part of' indicates a hidden word.
'CASTRO' can be found hidden inside 'ptolemaic astronomy'.
(Other definitions for castro that I've seen before include "Man of Cuba" , "Cuban leader, d. 2016" , "Fidel or Raul" , "Fidel or Ral" , "Old leader" .)