Best man enthralled by precious stone (7)
I believe the answer is:
optimal
'best' is the definition.
(optimal means the best)
'man enthralled by precious stone' is the wordplay.
'man' becomes 'Tim' (short for Timothy).
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'precious stone' becomes 'opal' (I've seen this before).
'tim' going into 'opal' is 'OPTIMAL'.
(Other definitions for optimal that I've seen before include "Am pilot being at best" , "Best possible" , "Ideal" , "couldn't be better" , "Most favouring" .)