Heads for tree, observing rare shade of trunk (5)
I believe the answer is:
torso
'trunk' is the definition.
(both can mean a person's chest)
'heads for tree observing rare shade of' is the wordplay.
'heads for' indicates taking the first letters.
The initial letters of 'tree observing rare shade of' is 'TORSO'.
(Other definitions for torso that I've seen before include "Body (excluding head, neck and limbs)" , "Roost on trunk?" , "Trunk; headless, limbless body" , "Body, minus its head and limbs" , "Roots out in trunk of the body" .)