Singer in bar given banknote, we hear (12)
I believe the answer is:
'singer' is the definition.
(I know that countertenor is a type of alto)
'in bar given banknote we hear' is the wordplay.
'in bar' becomes 'counter' (I've seen this before. I am not sure about the 'in' bit.).
'given' says to put letters next to each other.
'banknote' becomes 'tenner' (I've seen this before).
'we hear' shows a homophone (sound like).
'tenner' sounds like 'tenor'.
(Other definitions for countertenor that I've seen before include "Adult male with an alto voice" , "being highly vocal" , "he'll sing" , "Highest male singing voice" , "Singer" .)