Expert leaving to enter wine store (6)
I believe the answer is:
boffin
'expert' is the definition.
'leaving to enter wine store' is the wordplay.
'leaving' becomes 'off' (I've seen this in other clues).
'to enter' means one lot of letters goes inside another.
'wine store' becomes 'bin' (stand for storing wine bottles).
'off' put within 'bin' is 'BOFFIN'.
(Other definitions for boffin that I've seen before include "Brainbox" , "Scientist (informal)" , "Research scientist" , "Technical researcher" , "Back-room boy" .)