Mark follows crazy ruler to island (10)
I believe the answer is:
madagascar
'island' is the definition.
(island nation)
'mark follows crazy ruler' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'follows' is a charade indicator (letters next to each other).
'crazy' becomes 'mad' (synonyms).
'ruler' becomes 'aga' (I've seen this before).
'mad'+'aga'='madaga'
'scar' after 'madaga' is 'MADAGASCAR'.
'to' acts as a link.
(Other definitions for madagascar that I've seen before include "Island nation" , "film" , "Island republic in the Indian Ocean" , "country" , "Island republic of Africa" .)