Irritable chap taking in races (5)
I believe the answer is:
ratty
'irritable' is the definition.
(thesaurus)
'chap taking in races' is the wordplay.
'chap' becomes 'ray' (**).
'taking in' is an insertion indicator.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'ray' going around 'tt' is 'RATTY'.
(Other definitions for ratty that I've seen before include "Bad-tempered, irritable as a rodent" , "with pests" , "Irritable (colloq.)" , "Short" , "Mole's friend (The Wind in the Willows)" .)