Irritable like a horrible child having lost book (5)
I believe the answer is:
ratty
'irritable' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for ratty that I've seen before include "Waspish" , "Short-tempered" , "Moley's friend" , "Bad-tempered, irritable as a rodent" , "Irritable, peevish" .)