An animal corral hidden in the trees (6)
I believe the answer is:
aspens
'trees' is the definition.
(aspen is a kind of tree)
'an animal corral hidden in' is the wordplay.
'an animal' becomes 'ass' (term for a donkey).
'corral' becomes 'pen' (I've seen this in another clue).
'hidden in' indicates putting letters inside.
'ass' enclosing 'pen' is 'ASPENS'.
'the' is the link.
(Other definitions for aspens that I've seen before include "Trembling poplars" , "Kind of poplars with shivering leaves" , "Quivering poplars" , "Species of poplars with leaves that quiver" , "Trees that shiver, kind of poplars" .)