Slithery creature alarms dean horribly (10)
I believe the answer is:
salamander
'slithery creature' is the definition.
(salamanders slither around)
'alarms dean horribly' is the wordplay.
'horribly' indicates an anagram (a horribly wrong version of the letters).
'alarms'+'dean'='alarmsdean'
'alarmsdean' anagrammed gives 'SALAMANDER'.
(Other definitions for salamander that I've seen before include "portable stove" , "Mythical fire lizard" , "amphibious creature" , "Tailed amphibian" , "poker" .)