I'm composing a new tune - dance music (6)
I believe the answer is:
minuet
'dance music' is the definition.
(I've seen this in another clue)
'i'm composing a new tune' is the wordplay.
'composing' indicates anagramming the letters.
'a new' indicates an anagram.
'im' is an anagram of 'mi'.
'tune' with letters rearranged gives 'nuet'.
'mi'+'nuet'='MINUET'
(Other definitions for minuet that I've seen before include "18th-century dance" , "Stately old dance, takes only a minute" , "Stately court dance" , "Stately C 18 dance" , "Gracious old dance, just takes a minute" .)