Single line by individual (4)
I believe the answer is:
lone
'single' is the definition.
(synonyms)
'line by individual' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'by' is a charade indicator (letters next to each other).
'individual' becomes 'one' (an individual is one person).
'l'+'one'='LONE'
(Other definitions for lone that I've seen before include "The ... Ranger, fictional masked cowboy" , "Sole" , "He likes to operate by himself" , "Single and isolated" , "Companionless" .)