Mountainous island fit for climbing (4)
I believe the answer is:
elba
'mountainous island' is the definition.
'elba' can be an answer for 'island' (I've seen this before). I'm not certain of the 'mountainous' bit.
'fit for climbing' is the wordplay.
'fit' becomes 'able' ('able' can be similar in meaning to 'fit').
'for climbing' says the letters should be written backwards (in a down clue, letters climb up).
'able' in reverse letter order is 'ELBA'.
Can you help me to learn more?
(Other definitions for elba that I've seen before include "Where emperor was confined" , "Napoleonic prison" , "Napoleon's plaint: 'Able was I ere I saw . . . .'" , "Bale out of the Mediterranean island" , "part of Tuscany?" .)