Best man splitting stone (7)
I believe the answer is:
optimal
'best' is the definition.
(optimal means the best)
'man splitting stone' is the wordplay.
'man' becomes 'tim' (short for Timothy).
'splitting' indicates putting letters inside (some letters split a word into two).
'stone' becomes 'opal' (gemstone).
'tim' put within 'opal' is 'OPTIMAL'.
(Other definitions for optimal that I've seen before include "best results" , "Most desirable" , "Best or most favourable" , "Most favouring" , "Plot aim (anag) - most favourable" .)