It’s instrumental in Mozartian magic (5)
I believe the answer is:
flute
'mozartian magic' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'it's instrumental in' is the wordplay.
'it' becomes 't' (abbreviation. e.g. in 'tis).
'instrumental' becomes 'flue' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
't' inserted into 'flue' is 'FLUTE'.
Can you help me to learn more?
(Other definitions for flute that I've seen before include "High-pitched woodwind instrument" , "Blown" , "Musical instrument - wineglass" , "Champagne glass" , "See 20" .)