Fifty-one and single (4)
I believe the answer is:
lone
'single' is the definition.
(synonyms)
'fifty-one' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'l'+'one'='LONE'
'and' is the link.
(Other definitions for lone that I've seen before include "He likes to operate by himself" , "Solitary or single" , "The ... Ranger, fictional masked cowboy" , "Solitary, sounds like advance" , "Companionless" .)