Certain fire sign (3)
I believe the answer is:
leo
'certain fire sign' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leo that I've seen before include "sign of summer" , "Name taken by 13 popes, the last dying in 1903 aged 93" , "one of several popes" , "Fifth sign of the zodiac" , "Man" .)