So square brackets start to irritate? That's silly (7)
I believe the answer is:
asinine
'silly' is the definition.
(I've seen this before)
'so square brackets start to irritate?' is the wordplay.
'so' becomes 'as' (eg 'not as tall as me' means 'not so tall as me').
'square' becomes 'nine' (nine is a square number).
'brackets' is an insertion indicator (I've seen this in other clues).
'start to' says to take the initial letters.
The initial letter of 'irritate' is 'i'.
'as'+'nine'='asnine'
'asnine' placed around 'i' is 'ASININE'.
'that's' is the link.
(Other definitions for asinine that I've seen before include "Is inane to be foolish" , "Donkey-like" , "Stubborn" , "Idiotic" , "Foolish, silly" .)