Over a quarter are single (3)
I believe the answer is:
one
'are single' is the definition.
(similar in meaning)
'over a quarter' is the wordplay.
'over' becomes 'on' (physically on top of).
'a quarter' becomes 'e' ().
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "Single and undivided" , "Single, unitary" , "Single, sole" , "A certain; single person" , "Sole; number" .)