Bright boy put out of the race, so peevish (5)
I believe the answer is:
ratty
'peevish' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
'bright boy put out of the race' is the wordplay.
'bright boy put' becomes 'ray' (I can't justify this - if you can you should believe this answer much more).
'out of' is an insertion indicator (some letters on the outside of others).
'the race' becomes 'tt'.
'ray' enclosing 'tt' is 'RATTY'.
'so' is the link.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "Character created by Kenneth Grahame" , "Short" , "character on the river bank" , "Irritable - Mole's friend!" , "his friend being a dangerous driver" .)