Trees in the animal enclosure (6)
I believe the answer is:
aspens
'trees' is the definition.
(aspen is a kind of tree)
'in the animal enclosure' is the wordplay.
'in the' indicates putting letters inside.
'animal' becomes 'ass' (term for a donkey).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'ass' going around 'pen' is 'ASPENS'.
(Other definitions for aspens that I've seen before include "Trembling poplars" , "Species of poplars with leaves that quiver" , "Trembling trees" , "Trees with leaves that quiver" , "Quivering poplars" .)