Bar key painting? That's a slip (11)
I believe the answer is:
counterfoil
'a slip' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'bar key painting?' is the wordplay.
'bar' becomes 'counter' (as in the counter of a pub or bar).
'key painting?' becomes 'foil' (I can't justify this - if you can you should believe this answer much more).
'counter'+'foil'='COUNTERFOIL'
'that's' is the link.
Can you help me to learn more?
(Other definitions for counterfoil that I've seen before include "Record slip for cheque" , "in possession of purchaser" , "part of one cheque" , "Chequebook stub" , "record of financial transaction?" .)