Old man in one sort of tree has to climb another (5)
I believe the answer is:
maple
'old man in one sort of tree has to climb another' is the definition.
I can't tell whether this defines the answer.
'old man in one sort of tree has to climb' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'in' means one lot of letters goes inside another.
'one sort of tree has' becomes 'elm' (I can't explain this - if you can you should give a lot more credence to this answer).
'to climb' is a reversal indicator (I've seen 'climbing' mean this (in a down clue, letters climb up)).
'pa' placed into 'elm' is 'elpam'.
'elpam' back-to-front is 'MAPLE'.
Can you help me to learn more?
(Other definitions for maple that I've seen before include "Tree; sauce" , "Tree with winged fruits" , "Hard light-coloured wood used for furniture" , "Ample (anag.)" , "Tree; sugar from it" .)