Mastermind born during troubled reign (8)
I believe the answer is:
Here is my best explanation:
'mastermind' is the definition.
('engineer' can be a synonym of 'mastermind')
'born during troubled reign' is the subsidiary indication.
'born' becomes 'nee' (I've seen this before).
'during' is an insertion indicator.
'troubled' is an anagram indicator.
'reign' anagrammed gives 'engir'.
'nee' going into 'engir' is 'engineer'.
(Other definitions for engineer that I've seen before include "Cleverly bring about", "Deviser", "Machine designer, maker or operator", "Design and build", "One who designs, makes or runs motors", "Mechanic", "One uses science to solve practical problems", "One uses scientific knowledge to solve practical problems", "Bring about (an event); technologist" and "Chemical, civil, mechanical or electrical professional".)