One who questioned engineers in Rio quits (10)
I believe the answer is:
inquisitor
'one who questioned' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'engineers in rio quits' is the wordplay.
'engineers' indicates anagramming the letters (I've seen 'engineer' mean this).
'in'+'rio'+'quits'='inrioquits'
'inrioquits' is an anagram of 'INQUISITOR'.
Can you help me to learn more?
(Other definitions for inquisitor that I've seen before include "Searching questioner" , "Investigator, examiner" , "One questions" , "Exacting investigator" , "Torquemada, for example" .)