Its art is not laid out in squares (15)
I believe the answer is:
traditionalists
'squares' is the definition.
Although both the answer and definition are plural nouns, I can't understand how they can define each other.
'its art is not laid out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'its'+'art'+'is'+'not'+'laid'='itsartisnotlaid'
'itsartisnotlaid' is an anagram of 'TRADITIONALISTS'.
'in' acts as a link.
Can you help me to learn more?
(Another definition for traditionalists that I've seen is " old guard members".)