A process in calculating a high degree; it has no one returning (8)
I believe the answer is:
addition
'a process' is the definition.
(addition is a kind of process)
'calculating a high degree it has no one returning' is the wordplay.
'calculating a high' becomes 'ad' (I can't justify this - if you can you should give a lot more credence to this answer).
'degree' becomes 'd' (abbreviation for the degree in measuring angles).
'has' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'returning' says the letters should be written in reverse.
'no'+'i'='noi'
'noi' in reverse letter order is 'ion'.
'ad'+'d'+'it'+'ion'='ADDITION'
'in' is the link.
Can you help me to learn more?
(Other definitions for addition that I've seen before include "Calculation of the sum of two or more" , "Simple kind of arithmetic" , "an arithmetical process" , "'Increase, augmentation (8)'" , "Reckoning" .)