Introvert (5)
I believe the answer is:
loner
'introvert' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for loner that I've seen before include "One shunning company" , "Unsociable person" , "One not desiring companionship" , "One who avoids company" , "Solitary individual, prefers own company" .)