An opening permitted to a viewer (6)
I believe the answer is:
eyelet
'an opening' is the definition.
(eyelet is a kind of opening)
'permitted to a viewer' is the wordplay.
'permitted' becomes 'let' (to let something happen is to permit it).
'to a' says to put letters next to each other (I've seen this in other clues).
'viewer' becomes 'eye' (organ which sees or views).
'let' after 'eye' is 'EYELET'.
(Other definitions for eyelet that I've seen before include "Hole for a lace or cord" , "Shoe hole for lacing" , "Hole for fastener" , "Small lace-hole in shoe (6)" , "Fastening hole" .)