First thing about the mouth when imbibing one drink (8)
I believe the answer is:
original
'first' is the definition.
(similar in meaning)
'when imbibing one drink' is the wordplay.
'when' becomes 'orial' (I can't justify this - if you can you should believe this answer much more).
'imbibing' means one lot of letters goes inside another (imbibe can mean to drink or absorb).
'one drink' becomes 'gin' (gin is an alcoholic drink).
'orial' placed around 'gin' is 'ORIGINAL'.
'thing about the mouth' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for original that I've seen before include "Native" , "Existing from the start - quite new" , "The first, not a copy" , "Not derived or copied from anything else" , "Novel - archetype" .)