Rocky, mostly bleak place of exile (4)
I believe the answer is:
elba
'place of exile' is the definition.
(I've seen this before)
'rocky mostly bleak' is the wordplay.
'rocky' is an anagram indicator.
'mostly' means to remove the last letter.
'bleak' with its final letter taken away is 'blea'.
'blea' with letters rearranged gives 'ELBA'.
(Other definitions for elba that I've seen before include "Napoleonic prison" , "Napoleon's exile isle" , "Island where Napoleon was exiled" , "Exile isle for Napoleon" , "place for Napoleon" .)