Is unable to arrive at a harmonious composition (7)
I believe the answer is:
cantata
'harmonious composition' is the definition.
'cantata' can be an answer for 'composition' (I've seen this before). I'm not sure about the 'harmonious' bit.
'is unable to arrive at a' is the wordplay.
'is unable' becomes 'can't' (if you can't do something, you're unable to do it).
'to arrive' becomes 'ta' (this might be a standard abbreviation of which I'm unaware).
'at' says to put letters next to each other (I've seen this in other clues).
'ta' put after 'a' is 'ata'.
'cant'+'ata'='CANTATA'
Can you help me to learn more?
(Other definitions for cantata that I've seen before include "Musical piece" , "Short oratorio performed in concert" , "Vocal setting of a text (often religious)" , "Work for voice(s) and orchestra" , "Work for accompanied voice" .)