Singer with sense to prop up bar (12)
I believe the answer is:
countertenor
'singer' is the definition.
(countertenor is a kind of singer)
'sense to prop up bar' is the wordplay.
'sense' becomes 'tenor' (both can mean something's gist or meaning).
'to prop up' says to put letters next to each other.
'bar' becomes 'counter' (as in the counter of a pub or bar).
'tenor' after 'counter' is 'COUNTERTENOR'.
'with' acts as a link.
(Other definitions for countertenor that I've seen before include "Adult male with an alto voice" , "Highest male singing voice" , "he'll sing" , "Singer" , "being highly vocal" .)