It's a job being drunk mostly, right? (9)
I believe the answer is:
plasterer
'mostly right?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'it's a job being drunk' is the wordplay.
'it's a' becomes 'per' (eg 'one a day' means 'one per day').
'job being' becomes 'laster' (I can't explain this - if you can you should give a lot more credence to this answer).
'drunk' means one lot of letters goes inside another (inserted letters are drunk in).
'per' enclosing 'laster' is 'PLASTERER'.
Can you help me to learn more?
(Other definitions for plasterer that I've seen before include "may get one drunk?" , "One covering walls" , "Coverer of walls, ceilings, etc" , "Tradesman" , "Workman who skims along" .)