Tree snake seen regularly (5)
I believe the answer is:
Here is my best explanation:
'tree' is the definition.
(aspen is a kind of tree)
'snake seen regularly' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'seen' are 'en'.