Pigment? Unknown number to ring for its formula (4,5)
I believe the answer is:
zinc oxide
'pigment?' is the definition.
(thesaurus)
'unknown number to ring for its formula' is the wordplay.
I cannot quite understand how this works, but
'unknown' could be 'z' (algebra) and 'z' is present in the answer.
'number' could be 'xi' (XI is an example) and 'xi' is found within the answer.
'ring' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...
(Other definitions for zinc oxide that I've seen before include "white pigment" , "compound" .)