Solitary person - one between left and right (5)
I believe the answer is:
loner
'solitary person' is the definition.
(I've seen this before)
'one between left and right' is the wordplay.
'between' means one lot of letters goes inside another.
'left and right' becomes 'LR' (abbreviations for left and right).
'one' placed inside 'lr' is 'LONER'.
(Other definitions for loner that I've seen before include "Non-conformist" , "One preferring to act individually" , "Solitary person" , "One shunning society" , "One liking solitude" .)