An animal enclosure nestling in the trees (6)
I believe the answer is:
aspens
'trees' is the definition.
(aspen is a kind of tree)
'an animal enclosure nestling in the' is the wordplay.
'an animal' becomes 'ass' (I've seen this before).
'enclosure nestling' becomes 'pen' (I can't justify this - if you can you should give a lot more credence to this answer).
'in the' is an insertion indicator.
'ass' going around 'pen' is 'ASPENS'.
Can you help me to learn more?
(Other definitions for aspens that I've seen before include "Trees that shiver, kind of poplars" , "Species of poplars with leaves that quiver" , "Trembling trees" , "Poplar trees that shiver in the wind" , "Trembling poplars" .)