It’s at church, this irritation (4)
I believe the answer is:
itch
'this irritation' is the definition.
(thesaurus)
'it's at church' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'at' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'it'+'ch'='ITCH'
(Other definitions for itch that I've seen before include "Nagging wish" , "Hunger, longing" , "Strong desire to do something" , "Uncomfortable feeling" , "Longing; anag. of 19" .)