Write down a bit of a banjo tune (3)
I believe the answer is:
jot
'write down' is the definition.
(thesaurus)
'a bit of a banjo tune' is the wordplay.
'a bit of' says the answer is hidden in the clue.
'JOT' is hidden within 'banjo tune'.
(Other definitions for jot that I've seen before include "Write hurriedly" , "Write hastily - a very small amount" , "Write a short note" , "Set down briefly" , "Scrap" .)