Adult in German city, Mainz, ultimately getting a stroke of luck (7)
I believe the answer is:
bonanza
'stroke of luck' is the definition.
(I've seen this before)
'adult in german city mainz ultimately getting a' is the wordplay.
'adult' becomes 'a'.
'in' indicates putting letters inside.
'german city' becomes 'bonn' (I've seen this before).
'ultimately' says to take the final letters.
'getting' says to put letters next to each other.
The last letter of 'mainz' is 'z'.
'a' going into 'bonn' is 'bonan'.
'bonan'+'z'+'a'='BONANZA'
(Other definitions for bonanza that I've seen before include "Source of great and sudden wealth" , "Event creating a sudden increase in wealth" , "Source of wealth or good fortune" , "instant prosperity" , "Huge stroke of luck" .)