Best gem worn by a man (7)
I believe the answer is:
optimal
'best' is the definition.
(optimal means the best)
'gem worn by a man' is the wordplay.
'gem' becomes 'opal' (opal is a kind of gem).
'worn by' is an insertion indicator (outside letters are worn by those on the inside).
'a man' becomes 'Tim' (short for Timothy).
'opal' placed around 'tim' is 'OPTIMAL'.
(Other definitions for optimal that I've seen before include "Plot aim (anag) - most favourable" , "Ideal" , "Plot aim (anag) - best" , "couldn't be better" , "best results" .)