Sheep in tree, we hear! (3)
I believe the answer is:
ewe
'sheep' is the definition.
(ewe is a kind of sheep)
'in tree we hear' is the wordplay.
'in' says the answer is hidden in the clue.
'EWE' can be found hidden inside 'tree we hear'.
(Other definitions for ewe that I've seen before include "A palindromic sheep" , "Directions" , "She'll provide material to knit" , "Lamb's mother" , "Partner to ram" .)