Parisian bank ultimately cut interest completely (5)
I believe the answer is:
rivet
'interest completely' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'parisian bank ultimately cut' is the wordplay.
'parisian bank' becomes 'rive' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ultimately' indicates one should take the final letters.
The final letter of 'cut' is 't'.
'rive'+'t'='RIVET'
Can you help me to learn more?
(Other definitions for rivet that I've seen before include "Short metal fastener" , "join securely" , "Metal fastening pin" , "fix" , "Completely engross; pin" .)