Bright boy put out of the race, so peevish (5)
I believe the answer is:
ratty
'peevish' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can'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 give a lot more credence to this answer).
'out of' indicates putting letters inside.
'the race' becomes 'tt'.
'ray' going around '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" .)