Ian is up a mountain (5)
I believe the answer is:
sinai
'a mountain' is the definition.
(I've seen this before)
'ian is up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ian'+'is'='ianis'
'ianis' in reverse letter order is 'SINAI'.
(Other definitions for sinai that I've seen before include "Law handed down here" , "desert area" , "Biblical mount" , "Mountainous Egyptian peninsula" , "Part of 10 [EGYPT]" .)