Opera is occasionally a big deal (4)
I believe the answer is:
aida
'opera' is the definition.
(Verdi opera)
'occasionally a big deal' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'a'+'big'+'deal'='abigdeal'
The alternate letters of 'abigdeal' are 'AIDA'.
'is' acts as a link.
(Other definitions for aida that I've seen before include "She dies with Radams (Verdi)" , "slave of Covent Garden" , "Verdi's Egyptian opera" , "Operatic lady" , "eponymous heroine" .)