... if not overthrown, Charles le roi will imprison this group (2,4)
I believe the answer is:
or else
'if not' is the definition.
(I've seen this in another clue)
'overthrown charles le roi will imprison this group' is the wordplay.
'OR ELSE' is hidden back-to-front within the letters.
However, I am uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for or else that I've seen before include "Do it . . . . . . - an informal warning" , "Otherwise - that's a threat" , "a threatening couple of words" , "Statement that threatens" , "Otherwise - may be threatening" .)