Riemann exemplary contributing to addition (6)
I believe the answer is:
annexe
'addition' is the definition.
(annexe is a kind of addition)
'riemann exemplary contributing to' is the wordplay.
'contributing to' indicates a hidden word.
'ANNEXE' is hidden within 'riemann exemplary'.
(Other definitions for annexe that I've seen before include "Extra wing (on building)" , "Appropriate into one's territory" , "Wing of the main building" , "Building attached to a larger one" , "Extension, supplement" .)