Sheep heard in tree? (3)
I believe the answer is:
yew
'tree?' is the definition.
(type of tree)
'sheep heard' is the wordplay.
'sheep' becomes 'ewe' (a ewe is a female sheep).
'heard' indicates a 'sounds like' (homophone) clue.
'ewe' sounds like 'YEW'.
'in' acts as a link.
(Other definitions for yew that I've seen before include "Evergreen tree with dark needles and red berries" , "Leaving body" , "Evergreen shrub" , "Wood for bows" , "Tree associated with graveyards" .)