Unit involved in some unspecified periodic payment (7)
I believe the answer is:
annuity
'payment' is the definition.
(annuity is a kind of payment)
'unit involved in some unspecified periodic' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'unit' is 'nuit' which is located in the answer.
'some' could be 'any' (similar in meaning) and 'any' is found within the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for annuity that I've seen before include "Pension investment" , "Sum paid yearly" , "Pension income" , "Regular income from a fund" , "Yearly allowance" .)