Peevish as a bright lad put out of the race (5)
I believe the answer is:
ratty
'peevish' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'a bright lad put out of the race' is the wordplay.
'a bright lad 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 (some letters on the outside of others).
'the race' becomes 'tt'.
'ray' enclosing 'tt' is 'RATTY'.
'as' is the link.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "Cross - character in "The Wind in the Willows"" , "Moley's friend" , "In a bad temper" , "Characteristic of pest" , "with hump?" .)