Really peevish, like a deserter (5)
I believe the answer is:
ratty
'really peevish' is the definition.
I can't judge whether this defines the answer.
'deserter' is the wordplay.
I cannot quite see how this works, but
'deserter' could be 'rat' (both can mean a traitor) and 'rat' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'like a' acts as a link.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "his friend being a dangerous driver" , "Characteristic of pest" , "with pests" , "Waspish" , "Fictional mole's chum" .)