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' is an insertion indicator.
'french museum' becomes 'musee' ('museum' in French).
'st' placed inside 'musee' is 'MUST-SEE'.
(Other definitions for must-see that I've seen before include "Remarkable sight" , "Unmissable" .)