Sign over ä troubled mutual (6)
I believe the answer is:
umlaut
'sign over' is the definition.
'umlaut' can be an answer for 'sign' (I have seen 'Diacritical sign ' mean 'umlaut' so perhaps 'sign' could also mean 'umlaut'). I am unsure of the 'over' bit.
'a troubled mutual' is the wordplay.
'a troubled' indicates an anagram.
'mutual' is an anagram of 'UMLAUT'.
Can you help me to learn more?
(Other definitions for umlaut that I've seen before include "Mark over some German letters" , "Dots over letter (indicating vowel change)" , "Mark put over a vowel in German to indicate a different quality" , "German accent" , "Two dots put over a vowel in German" .)