Series of fifty-one points
I believe the answer is:
line
'series' is the definition.
(line is a kind of series)
'fifty-one points' is the wordplay.
'fifty one' becomes 'LI' (Roman numerals).
'points' becomes 'ne' (North and East).
'li'+'ne'='LINE'
'of' acts as a link.
(Other definitions for line that I've seen before include "Family" , "An American queue" , "Railway track; policy" , "column" , "Long mark; direction" .)