Some could help a man climbing a tree (5)
I believe the answer is:
maple
'a tree' is the definition.
(maple is a kind of tree)
'some could help a man climbing' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'elpam' is hidden within 'could help a man'.
'elpam' back-to-front is 'MAPLE'.
(Other definitions for maple that I've seen before include "Cathedral city in Devon" , "Its leaf is the emblem of 8 down" , "Type of syrup" , "plant" , "a lot of these in Canada" .)