Miller regularly climbing a tree (3)
I believe the answer is:
elm
'a tree' is the definition.
(I know that elm is a type of tree)
'miller regularly climbing' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
The alternate letters of 'miller' are 'mle'.
'mle' back-to-front is 'ELM'.
(Other definitions for elm that I've seen before include "wooden" , "Victim of fungus" , "Source of timber" , "Hardwood tree" , "Wood in pelmet" .)