Celebrity royal: a snake! (5)
I believe the answer is:
viper
'a snake' is the definition.
(viper is a kind of snake)
'celebrity royal' is the wordplay.
'celebrity' becomes 'vip'.
'royal' becomes 'er' (I've seen this in other clues).
'vip'+'er'='VIPER'
(Other definitions for viper that I've seen before include "Deadly-venomous snake" , "Adder" , "Malicious type" , "A poisonous snake or treacherous person" , "crawler" .)