Loner (6)
I believe the answer is:
hermit
'loner' is the definition.
(hermit is a kind of loner)
This is the entire clue.
(Other definitions for hermit that I've seen before include "Ascetic who leads an isolated life" , "Solitary person, anchorite" , "A recluse or a crab" , "Kind of crab that lives in a cast-off shell" , "Solitary dweller" .)