Not quite a big boy, nevertheless a brute! (5)
I believe the answer is:
bison
'brute' is the definition.
(I've seen this before)
'not quite a big boy' is the wordplay.
'not quite' means to remove the last letter (most but not all of the word).
'boy' becomes 'son' (male child).
'big' with its final letter taken away is 'bi'.
'bi'+'son'='BISON'
'nevertheless a' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for bison that I've seen before include "US prairie cattle" , "Large shaggy-maned animal" , "North American buffalo" , "Big beast" , "It's a buffalo in America" .)