Swan taking donkey round trees (6)
I believe the answer is:
aspens
'trees' is the definition.
(aspen is a kind of tree)
'swan taking donkey round' is the wordplay.
'swan' becomes 'pen' (pen is a kind of swan).
'taking donkey' becomes 'ass' (ass is a term for a donkey. I am not sure about the 'taking' bit.).
'round' indicates putting letters inside.
'pen' placed inside 'ass' is 'ASPENS'.
Can you help me to learn more?
(Other definitions for aspens that I've seen before include "Kind of poplars with shivering leaves" , "Trembling poplars" , "Species of poplars with leaves that quiver" , "Quivering poplars" , "The leaves of the trees flutter in the slightest breeze" .)