Deadly time to stab the French prince (6)
I believe the answer is:
lethal
'deadly' is the definition.
(I know that deadly can be written as lethal)
'time to stab the french prince' is the wordplay.
'time' becomes 't' (abbreviation).
'to stab' indicates putting letters inside (some letters stab themselves into others).
'the french' becomes 'le' ('the' in French).
'prince' becomes 'hal' (name for Prince Henry in Shakespeare's histories).
'le'+'hal'='lehal'
't' put inside 'lehal' is 'LETHAL'.
(Other definitions for lethal that I've seen before include "Noxious" , "Murderous" , "describing a killer?" , "Able to cause death" , "bringing down the curtain" .)