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'm 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' means one lot of letters goes inside another.
'a rut' becomes 'be' (I can't explain this - if you can you should give a lot more credence to this answer).
'rut' put within '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 ____ !"" .)