Odd number, grammatically (8)
I believe the answer is:
I believe this clue is a double definition.
'odd' is the first definition.
(I know that odd can be written as singular)
'number grammatically' is the second definition.
(in grammar, number can mean singular or plural)
(Other definitions for singular that I've seen before include "Unusual - not plural" , "number" , "Referring to one thing" , "Denoting one" , "Unusual, striking, or unique" .)