Small island the very thing for recluse (6)
I believe the answer is:
hermit
'for recluse' is the definition.
(synonyms)
'small island the very thing' is the wordplay.
'small island' becomes 'Herm' (Channel Island).
'the very thing' becomes 'it' ('that's it!').
'herm'+'it'='HERMIT'
(Other definitions for hermit that I've seen before include "Kind of crab that lives in a cast-off shell" , "A recluse or a crab" , "Anchorite, person who has withdrawn from society" , "One lives an isolated religious life" , "One who lives in solitude, like kind of crab" .)