Supposed musical sheep flipped (8)
I believe the answer is:
putative
'supposed' is the definition.
('putative' can be a synonym of 'supposed')
'musical sheep flipped' is the wordplay.
'musical' becomes 'evita' (I've seen this before).
'sheep' becomes 'tup' (tup is a kind of sheep).
'flipped' says the letters should be written in reverse.
'evita'+'tup'='evitatup'
'evitatup' in reverse letter order is 'PUTATIVE'.
(Other definitions for putative that I've seen before include "Generally considered or reputed to be" , "Supposed or reputed" , "Supposed or thought to be" , "Supposed, generally considered such" , "Supposed - tie up vat (anag)" .)