Extremely silly Mozart's first fake composition (8)
I believe the answer is:
symphony
'composition' is the definition.
(I've seen this before)
'extremely silly mozart's first fake' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'first' indicates taking the first letters.
'fake' becomes 'phony' ('phony' can be a synonym of 'fake'**).
'silly' with its middle removed is 'sy'.
The initial letter of 'mozarts' is 'm'.
'sy'+'m'+'phony'='SYMPHONY'
(Other definitions for symphony that I've seen before include "Large orchestral piece" , "Grand orchestral composition" , "Eg, the Eroica" , "Large orchestral composition" , "See 17" .)