Mary's pet (4)
I believe the answer is:
lamb
'mary's pet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lamb that I've seen before include "Agnus Dei - . . . . of God" , "Young shee" , "essayists?" , "Noted essayist - symbol of innocence" , "Charles ---, essayist; meek animal" .)