Large instrument demonstrated in public performance getting magnificent support (7,5)
I believe the answer is:
concert grand
'large instrument' is the definition.
'concert grand' can be an answer for 'instrument' (concert grand is a kind of instrument). I'm not certain of the 'large' bit.
'in public performance getting magnificent support' is the wordplay.
I cannot quite understand how this works, but
'performance' could be 'concert' (concert is a kind of performance) and 'concert' is found in the answer.
'magnificent' could be 'grand' (I've seen this before) and 'grand' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'demonstrated' acts as a link.
Can you help me to learn more?
(Another definition for concert grand that I've seen is " Large piano".)