They have square faces (5)
I believe the answer is:
cubes
'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps you can see an association between them that I can't see?
'square faces' is the wordplay.
'square' becomes 'cube' (I have seen 'Solid square' mean 'cube' so perhaps 'square' could also mean 'cube').
'faces' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cube'+'s'='CUBES'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for cubes that I've seen before include "Solid body with six equal square faces" , "1, 8, 27 etc" , "blocks" , "figures" , "die? More than one" .)