That’s uncle starting the barn dance (5)
I believe the answer is:
samba
'dance' is the definition.
(Brazilian dance)
'that's uncle starting the barn' is the wordplay.
'that's uncle' becomes 'sam' (Uncle Sam is an example).
'starting the barn' becomes 'ba' (I can't justify this - if you can you should believe this answer much more).
'sam'+'ba'='SAMBA'
Can you help me to learn more?
(Other definitions for samba that I've seen before include "Brazilian dance" , "it was launched in Brazil" , "moves from Brazil" , "This ambassador's > brisk steps" , "Latin dance" .)