Number to sing — something positive or negative? (8)
I believe the answer is:
trillion
'number' is the definition.
(trillion is a kind of number)
'sing something positive or negative?' is the wordplay.
'sing' becomes 'trill' (trilling is a kind of singing).
'something positive or negative?' becomes 'ion' (type of atom).
'trill'+'ion'='TRILLION'
'to' acts as a link.
(Other definitions for trillion that I've seen before include "Very many" , "Very large power of 10" , "enormous figure" , "1 and 18 zeros" , "A million multiplied by twice itself" .)