Not a viable frame is to be seen (7)
I believe the answer is:
visible
'be seen' is the definition.
(thesaurus)
'not a viable frame is' is the wordplay.
'not' is a deletion indicator.
'frame' means one lot of letters goes inside another.
'viable' with 'a' removed is 'vible'.
'vible' placed around 'is' is 'VISIBLE'.
'to' acts as a link.
(Other definitions for visible that I've seen before include "Obvious to the eye" , "Able to be seen" , "Showing up" , "Perceptible by the eye" , "Apparent" .)