Moral primer replaced by satire (8)
I believe the answer is:
bestiary
'moral primer' is the definition.
(moralising medieval work)
'replaced by satire' is the wordplay.
'replaced' indicates anagramming the letters.
'by'+'satire'='bysatire'
'bysatire' with letters rearranged gives 'BESTIARY'.
(Other definitions for bestiary that I've seen before include "medieval treatise" , "Medieval collection of descriptions of animals" , "Medieval book of animals for sybarite" , "Allegorical work" , "Medieval collection of animal descriptions for sybarite" .)