Philosopher climbing old mountain outside Turkish capital (5)
I believe the answer is:
plato
'philosopher' is the definition.
(Plato is an example)
'climbing old mountain outside turkish capital' is the wordplay.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mountain' becomes 'alp' (the Alps are a mountain range).
'outside' is an insertion indicator.
'capital' indicates taking the first letters.
The initial letter of 'turkish' is 't'.
'o'+'alp'='oalp'
'oalp' placed around 't' is 'otalp'.
'otalp' in reverse letter order is 'PLATO'.
(Other definitions for plato that I've seen before include "A plot (anag) (known to Aristotle?)" , "Republican writer?" , "Moon crater named after Greek philosopher" , "Ancient Greek philosopher" , "Ancient Athenian philosopher, pupil of Socrates" .)