Deterioration of nickel regularly occurring after ten years (9)
I believe the answer is:
decadence
'deterioration' is the definition.
(thesaurus)
'after ten years' is the wordplay.
'after' means one lot of letters go next to another.
'ten' becomes 'decad' (I've seen this in another clue).
'years' becomes 'ence' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'decad' put next to 'ence' is 'DECADENCE'.
'of nickel regularly occurring' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for decadence that I've seen before include "Degeneration" , "Decline in quality" , "State of immorality and self-indulgence" , "depravity" , "Debauchery, degeneracy" .)