They are all right in the square (6)
I believe the answer is:
angles
'they' is the definition.
Both the definition and answer are plural nouns.
Maybe they are linked in a way I don't understand?
'are all right in the square' is the wordplay.
'are' becomes 'ange' (I can't justify this - if you can you should believe this answer much more).
'right' suggests the final letters (the letter on the right of the word).
'in' is an insertion indicator.
'the square' becomes 's' (abbreviation).
The final letter of 'all' is 'l'.
'ange' enclosing 'l' is 'angle'.
'angle'+'s'='ANGLES'
Can you help me to learn more?
(Other definitions for angles that I've seen before include "Ways of approaching" , "those who met Pope Gregory" , "crooks" , "eg, Right, acute, obtuse" , "Space between two lines at a corner" .)