Island very keen on a hydrogen vehicle? (10)
I believe the answer is:
madagascar
'island' is the definition.
(island nation)
'very keen on a hydrogen vehicle?' is the wordplay.
'very keen' becomes 'mad' (I've seen this in other clues).
'on' says to put letters next to each other.
'hydrogen' becomes 'gas' (hydrogen is a gas).
'vehicle?' becomes 'car' (car is a kind of vehicle).
'mad'+'a'+'gas'+'car'='MADAGASCAR'
(Other definitions for madagascar that I've seen before include "Large island off the east coast of Africa" , "foreign location" , "Part of Africa" , "Island off SE Africa" , "country" .)