Regularly we climb tree (3)
I believe the answer is:
elm
'tree' is the definition.
(elm tree is a kind of tree)
'regularly we climb' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'we'+'climb'='weclimb'
The alternate letters of 'weclimb' are 'ELM'.
(Other definitions for elm that I've seen before include "I branch out" , "wooden" , "Gray's rugged tree (Elegy)" , "Forest tree" , "Tall shade tree" .)