Only fifty-one (4)
I believe the answer is:
'only' is the definition.
(I know that only can be written as lone)
'fifty-one' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
(Other definitions for lone that I've seen before include "Companionless" , "Solitary, sounds like advance" , "Solitary or single" , "Unaccompanied" , "Sole" .)