Tailed zebras every so often, arriving at trees (6)
I believe the answer is:
alders
'trees' is the definition.
(alder is a kind of tree)
'tailed zebras every so often' is the wordplay.
'every so often' means one should take alternating letters.
'tailed'+'zebras'='tailedzebras'
The alternating letters of 'tailedzebras' are 'ALDERS'.
'arriving at' is the link.
(Other definitions for alders that I've seen before include "Al's red trees, strangely" , "Catkin-bearing trees" , "Trees with toothed leaves" , "Trees for red Sal" , "Trees of the birch family" .)