Peephole for a viewer to lease (6)
I believe the answer is:
eyelet
'peephole' is the definition.
(I've seen this before)
'a viewer to lease' is the wordplay.
'a viewer' becomes 'eye' (I've seen this before).
'to lease' becomes 'let' ('let' can be a synonym of 'lease').
'eye'+'let'='EYELET'
'for' is the link.
(Other definitions for eyelet that I've seen before include "Small hole through which lace passes" , "Hole for a lace or cord" , "Small reinforced hole" , "Grommet for a shoe" , "Metal ring reinforcing a small round hole in fabric" .)