Round openings with no actual fit, regularly (5)
I believe the answer is:
oculi
'round openings' is the definition.
'no actual fit regularly' is the wordplay.
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'actual'+'fit'='actualfit'
The alternate letters of 'actualfit' are 'culi'.
'o'+'culi'='OCULI'
'with' is the link.
(Other definitions for oculi that I've seen before include "Eyes; eye-like opening in architecture, like a round window" , "Circular windows" .)