Unmissable saint in French museum (4-3)
I believe the answer is:
must-see
'unmissable' is the definition.
(I've seen this before)
'saint in french museum' is the wordplay.
'saint' becomes 'st' (abbreviation).
'in' indicates putting letters inside.
'french museum' becomes 'musee' ('museum' in French).
'st' put into 'musee' is 'MUST-SEE'.
(Other definitions for must-see that I've seen before include "Remarkable sight" , "Unmissable" .)