Signifier of five divided by zero for ruling family (7)
I believe the answer is:
romanov
'for ruling family' is the definition.
(Russian royal family)
'signifier of five' is the wordplay.
'signifier' becomes 'roman' (I am not sure about this - if you are sure you should believe this answer much more).
'of' becomes 'o''.
'five' becomes 'V' (Roman numeral).
'roman'+'o'+'v'='ROMANOV'
'divided by zero' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for romanov that I've seen before include "Member of the Russian royal family, 1613 to 1917" , "(Member of) Russian imperial dynasty" , "Russian royal dynasty" , "Family name of tsars" , "Russian imperial family name" .)