Tooth cooler with filling, ultimately (4)
I believe the answer is:
fang
'tooth' is the definition.
(I know that fang is a type of tooth)
'cooler with filling ultimately' is the wordplay.
'cooler' becomes 'fan' (something which cools).
'with' is a charade indicator (letters next to each other).
'ultimately' says to take the final letters.
The final letter of 'filling' is 'g'.
'fan'+'g'='FANG'
(Other definitions for fang that I've seen before include "A long sharp tooth as of wolf" , "Venomous tooth" , "possible poisoner" , "nipper" , "Dracula's tooth?" .)