In essence, a tree swallow (3)
I believe the answer is:
eat
'swallow' is the definition.
(I've seen this in other clues)
'in essence a tree' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EAT' is hidden within 'essence a tree'.
(Other definitions for eat that I've seen before include "Scoff" , "tolerate in diet" , "get stuck in" , "Worry" , "Munch" .)