Best possible period not quite set in stone (7)
I believe the answer is:
optimal
'best possible' is the definition.
(optimal means the best)
'period not quite set in stone' is the wordplay.
'period' becomes 'time' (time is a kind of period).
'not quite' means to remove the last letter.
'set in' means one lot of letters goes inside another.
'stone' becomes 'opal' (gemstone).
'time' with its last letter taken away is 'tim'.
'tim' placed inside 'opal' is 'OPTIMAL'.
(Other definitions for optimal that I've seen before include "as good as can be" , "Plot aim (anag) - most favourable" , "Best or most favourable" , "Plot aim (anag) - best" , "best results" .)