Irritable as overrun with vermin? (5)
I believe the answer is:
ratty
I believe this clue is a double definition.
'irritable' is the first definition.
(I've seen this before)
'overrun with vermin?' is the second definition.
(I've seen this in another clue)
'as' is the link.
(Other definitions for ratty that I've seen before include "with hump?" , "Moley's friend" , "Irritable (informal)" , "Irritable, peevish" , "Cross - character in "The Wind in the Willows"" .)