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' indicates a hidden word (some parts of the word are used).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'elpam' can be found hidden inside 'could help a man'.
'elpam' written backwards gives '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" .)