Anchor that may be square? (4)
I believe the answer is:
root
I believe this is a double definition.
'anchor' is the first definition.
(thesaurus)
'square?' is the second definition.
(square root is a type of mathematical root)
'that may be' acts as a link.
(Other definitions for root that I've seen before include "grub" , "Source; firmly establish" , "Lowest part of plant" , "Basic source or origin" , "could be square" .)