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' (thesaurus). I am not certain of the rest 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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'rut' going into '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 ____ !"" .)