This beastly fellow will never be in a rut! (5)
I believe the answer is:
brute
'this beastly fellow will never' is the definition.
'brute' can be an answer for 'beastly' (brute can mean beastly or brutish). I am not sure about the remainder of the definition.
'be in a rut' is the wordplay.
'be' becomes 'rut' (rutting is a kind of being).
'in' is an insertion indicator.
'a rut' becomes 'be' (I can't explain this - if you can you should believe this answer much more).
'rut' going inside 'be' is 'BRUTE'.
Can you help me to learn more?
(Other definitions for brute that I've seen before include "Caliban" , "Vicious type" , "Wild animal" , "Cruelly rapacious person" , "Beast - "Et tu ____ !"" .)