Party in back bar (5)
I believe the answer is:
revel
'party in back bar' is the definition.
'revel' can be an answer for 'party' (I've seen this before). I am not sure about the 'in back bar' bit.
'party in back' is the wordplay.
'party' becomes 'l'.
'in' becomes 'ever' (I can't justify this - if you can you should believe this answer much more).
'back' is a reversal indicator.
'l'+'ever'='lever'
'lever' written backwards gives 'REVEL'.
Can you help me to learn more?
(Other definitions for revel that I've seen before include "Carouse" , "Be festive, have a good time" , "Riotous feast" , "Rejoice" , "Celebrate noisily" .)