Other tune's composed in addition (9)
I believe the answer is:
thereunto
'in addition' is the definition.
(I've seen this before)
'other tune's composed' is the wordplay.
'composed' indicates anagramming the letters.
'other'+'tune'='othertune'
'othertune' is an anagram of 'THEREUNTO'.
(Other definitions for thereunto that I've seen before include "to that place" , "To that (archaic)" , "To that (formal)" , "as well" , "Almost obsolete word meaning 'to that'; formally, 'in addition'" .)