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 "Effect this professional man", "plan", "One who designs, makes or runs motors", "Operator of a railway locomotive", "control", "Skilled professional - electrical, civil or mechanical", "He may be civil, mechanical or electrical", "Person who uses scientific knowledge to solve practical problems", "Cleverly bring about" and "Practical technologist".)