Trees like female swans (6)
I believe the answer is:
aspens
'trees' is the definition.
(aspen is a kind of tree)
'like female swans' is the wordplay.
'like' becomes 'as'.
'female swans' becomes 'pens' (a female swan is a pen).
'as'+'pens'='ASPENS'
(Other definitions for aspens that I've seen before include "Trembling trees" , "Trees that shiver, kind of poplars" , "The leaves of the trees flutter in the slightest breeze" , "Species of poplars with leaves that quiver" , "Trembling poplars" .)