Greek astronomer Ptolemy ultimately finding ideal situation (6)
I believe the answer is:
theory
'ideal situation' is the definition.
(I've seen this before)
'greek astronomer ptolemy ultimately' is the wordplay.
'greek' becomes 'theo' (common Greek name).
'ultimately' indicates one should take the final letters.
The last letters of 'astronomer ptolemy' are 'ry'.
'theo'+'ry'='THEORY'
'finding' acts as a link.
(Other definitions for theory that I've seen before include "Explanation of a phenomenon" , "The presumption" , "Scientific conjecture" , "Set of ideas formulated to explain something" , "can provide an explanation" .)