Solitary, heartless internment on island (6)
I believe the answer is:
hermit
'solitary' is the definition.
(thesaurus)
'heartless internment on island' is the wordplay.
'heartless' means to remove the middle letters.
'on' is a charade indicator (letters next to each other).
'island' becomes 'herm' (**).
'internment' with its middle taken out is 'it'.
'it' after 'herm' is 'HERMIT'.
(Other definitions for hermit that I've seen before include "Solitary dweller" , "Ascetic who leads an isolated life" , "One who lives in solitude, like kind of crab" , "Liver in solitude" , "Anchorite or religious recluse" .)