Most strict testers finish off examination with a twist (8)
I believe the answer is:
sternest
'most strict' is the definition.
('stern' can be a synonym of 'strict')
'testers finish off examination with a twist' is the wordplay.
'finish off' says to take the final letters (I've seen 'finishing off' mean this).
'with a twist' is an anagram indicator (I've seen this in other clues).
The final letter of 'examination' is 'n'.
'testers'+'n'='testersn'
'testersn' anagrammed gives 'STERNEST'.
(Other definitions for sternest that I've seen before include "Most strict and demanding" , "Most unfriendly" , "Strictest or grimmest" , "Most serious and unrelenting" , "Most severe and unrelenting" .)