Dangerous influence monarch introduced to all French (11)
I believe the answer is:
treacherous
'dangerous' is the definition.
('treacherous' can be similar in meaning to 'dangerous')
'influence monarch introduced to all french' is the wordplay.
'influence' becomes 'reach'.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'introduced to' is an insertion indicator.
'all french' becomes 'tous' (French for 'all').
'reach'+'er'='reacher'
'reacher' inserted into 'tous' is 'TREACHEROUS'.
(Other definitions for treacherous that I've seen before include "Dangerous sort of ground" , "Perfidious" , "Ready to betray" , "Unreliable" , "Disloyal; dangerous" .)