German mark (6)
I believe the answer is:
umlaut
'german mark?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for umlaut that I've seen before include "Diacritical sign" , "Two dots put over a vowel in German" , "In German, written mark indicating change in vowel pronunciation" , "Mark over some vowels (in, eg, German)" , "Mark indicating changed vowel value in German" .)