Number involved in tree-climbing exercise (5)
I believe the answer is:
exert
'exercise' is the definition.
(I've seen this before)
'number involved in tree-climbing' is the wordplay.
'number' becomes 'x' (X is an example**).
'involved in' means one lot of letters goes inside another.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'x' going into 'tree' is 'trexe'.
'trexe' in reverse letter order is 'EXERT'.
(Other definitions for exert that I've seen before include "Work very hard" , "Strain" , "Bring into use, say strength" , "Put to use, vigorously" , "Bring (force) to bear" .)