Anyone can disguise irritation (9)
I believe the answer is:
annoyance
'irritation' is the definition.
('annoyance' can be a synonym of 'irritation')
'anyone can disguise' is the wordplay.
'disguise' is an anagram indicator (I've seen 'disguised' mean this).
'anyone'+'can'='anyonecan'
'anyonecan' with letters rearranged gives 'ANNOYANCE'.
(Other definitions for annoyance that I've seen before include "Irritation, vexation" , "Source of anger" , "Nuisance" , "pest" , "Feeling of slight anger" .)