Snake seen off and on in tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'snake seen off and on' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'off and on' means one should take alternating letters (letters used at regular intervals).
The alternating letters of 'seen' are 'en'.
'asp'+'en'='ASPEN'
'in' acts as a link.
(Other definitions for aspen that I've seen before include "Tree that quivers" , "Tree with shivering leaves" , "Growth" , "Tremulous poplar" , "somewhere in Colorado" .)