Irritable truants try getting even (5)
I believe the answer is:
ratty
'irritable' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "Irritable (informal)" , "Characteristic of pest" , "Mole's friend (The Wind in the Willows)" , "Bad tempered" , "water vole, literally" .)