Solitary large number (4)
I believe the answer is:
lone
'solitary' is the definition.
(I know that solitary can be written as lone)
'large number' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'number' becomes 'one' (one is a kind of number).
'l'+'one'='LONE'
(Other definitions for lone that I've seen before include "Solitary or single" , "The ... Ranger, fictional masked cowboy" , "Sole" , "He likes to operate by himself" , "Companionless" .)