A German saint meets a German scientist (8)
I believe the answer is:
einstein
'scientist' is the definition.
(I know that Einstein is a physicist)
'a german saint meets a german' is the wordplay.
'a german' becomes 'ein' ('a' in German).
'saint' becomes 'st' (abbreviation).
'meets' is a charade indicator (letters next to each other).
'a german' becomes 'ein' ('a' in German).
'ein'+'st'+'ein'='EINSTEIN'
(Other definitions for einstein that I've seen before include "19 [BRAINY] fellow" , "Bavarian" , "Set nine up for his theory of relativity" , "Famous physicist, mathematician" , "Clever clogs finding" .)