A number remains -- so hurry! (4)
I believe the answer is:
dash
'hurry' is the definition.
(dashing is a kind of hurrying)
'a number remains' is the wordplay.
'a number' becomes 'd' (D is an example**).
'remains' becomes 'ash' (I've seen this before).
'd'+'ash'='DASH'
'so' is the link.
(Other definitions for dash that I've seen before include "Stroke < > to frustrate" , "DOG" , "Smash - bother! - make a run for it" , "Run hurriedly; hyphen" , "Kill off (hopes)" .)