Mountain climbing naked pianist (5)
I believe the answer is:
sinai
'mountain' is the definition.
(I've seen this before)
'climbing naked pianist' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'naked' indicates a hidden word (with the letters around the hidden word taken off).
'ianis' can be found hidden inside 'pianist'.
'ianis' backwards is 'SINAI'.
(Other definitions for sinai that I've seen before include "11 Across peninsula" , "Mount where Moses received God's law" , "desert area" , "Deserted area" , "Part of 10 [EGYPT]" .)