It's totally different from a weasel! (5)
I believe the answer is:
stoat
'a weasel' is the definition.
(I know that stoat is a type of Mustela erminea)
'it's totally different from' is the wordplay.
'totally different' indicates anagramming the letters (I've seen this in other clues).
'from' becomes 'oat' (I can't explain this - if you can you should believe this answer much more).
'ts' anagrammed gives 'st'.
'st'+'oat'='STOAT'
Can you help me to learn more?
(Other definitions for stoat that I've seen before include "Source of ermine" , "European ermine" , "Small carnivore" , "Small carnivorous mammal of the weasel family" , "Eating rabbit, he" .)