1 + 50 + 1 = 1? (5)
I believe the answer is:
alone
'1?' is the definition.
(someone alone is a single person)
'1 50 1' is the wordplay.
'1' becomes 'a' (both can mean one).
'50' becomes 'L' (roman numeral).
'1' becomes 'one' (synonyms).
'a'+'l'+'one'='ALONE'
(Other definitions for alone that I've seen before include "without another" , "Solo" , "Note of charges" , "Isolated; exclusively" , "Without others being involved" .)