Irritable having mislaid pencil-set (9)
I believe the answer is:
splenetic
'irritable' is the definition.
(I've seen this before)
'mislaid pencil-set' is the wordplay.
'mislaid' indicates an anagram.
'pencil'+'set'='pencilset'
'pencilset' anagrammed gives 'SPLENETIC'.
'having' acts as a link.
(Other definitions for splenetic that I've seen before include "testy" , "Often cross" , "Irritable" , "Peevish" , "Bad-tempered, spiteful" .)