Irritable agent ringing down (6)
I believe the answer is:
snappy
'irritable' is the definition.
(thesaurus)
'agent ringing down' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'ringing' is an insertion indicator.
'down' becomes 'nap' (close in meaning in sense of hairy surface).
'spy' going around 'nap' is 'SNAPPY'.
(Other definitions for snappy that I've seen before include "without hesitation" , "Instantaneous" , "Irritable, curt; stylish" , "Cleverly concise; stylish" , "as a croc?" .)