One may put his suit in a case - or vice versa (8)
I believe the answer is:
litigant
'put his suit in a case or vice versa' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one' is the wordplay.
I cannot really understand how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'may' acts as a link.
Can you help me to learn more?
(Other definitions for litigant that I've seen before include "Person involved in a lawsuit" , "I'm in court" , "One who goes to law" , "Plaintiff" , "Party in lawsuit" .)