Mountain in Japan, utterly fabulous, initially, to climb (4)
I believe the answer is:
fuji
'mountain' is the definition.
(I have seen 'Japanese mountain ' mean 'fuji' so perhaps 'mountain' could also mean 'fuji')
'in japan utterly fabulous initially to climb' is the wordplay.
'initially' suggests taking the first letters.
'to climb' says the letters should be written in reverse (I've seen 'climbs' mean this).
The initial letters of 'in japan utterly fabulous' is 'ijuf'.
'ijuf' back-to-front is 'FUJI'.
(Other definitions for fuji that I've seen before include "Extinct Japanese volcano - cherry tree" , "Sacred volcano of Japan" , "Sacred Japanese peak" , "Japan's highest peak" , "Extinct volcano in south central Honshu" .)