Philosopher's occupational therapy: mountain climbing (5)
I believe the answer is:
plato
'philosopher's' is the definition.
(Plato is an example)
'occupational therapy mountain climbing' is the wordplay.
'occupational therapy' becomes 'ot' (abbreviation for occupational therapy).
'mountain' becomes 'alp' (the Alps are a mountain range).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'ot'+'alp'='otalp'
'otalp' written backwards gives 'PLATO'.
(Other definitions for plato that I've seen before include "Ancient Athenian philosopher, pupil of Socrates" , "Moon crater named after Greek philosopher" , "Philosopher of ancient Greece" , "Greek character" , "Greek philosopher (The Republic)" .)