Leading man drinking wine in it (9)
I believe the answer is:
important
'leading' is the definition.
(I've seen this before)
'man drinking wine in it' is the wordplay.
'drinking' is an insertion indicator.
'wine' becomes 'port' (port is a kind of wine**).
'in' means one lot of letters goes inside another.
'man' placed around 'port' is 'mportan'.
'mportan' going within 'it' is 'IMPORTANT'.
(Other definitions for important that I've seen before include "essence" , "Meaningful" , "Crucial" , "It's vital" , "Historic" .)