Some number in edges on equilaterals, squares (5)
I believe the answer is:
nines
'squares' is the definition.
(I've seen this before)
'some number in edges on equilaterals' is the wordplay.
'some number' becomes 'n' (n can represent any number).
'edges on' means to remove the middle letters.
'equilaterals' with its centre removed is 'es'.
'n'+'in'+'es'='NINES'
(Other definitions for nines that I've seen before include "9s" , "Dressed to these and you're well-turned-out" , "Numbers to which one may be dressed up?" , "Very smart if dressed to these" , "Dressed to these is to be perfectly turned out" .)