The French agree, accepting hotel with fatal consequences (8)
I believe the answer is:
lethally
'with fatal consequences' is the definition.
'the french agree accepting hotel' is the wordplay.
'the french' becomes 'le'.
'agree' becomes 'tally' ('tally' can be a synonym of 'agree'**).
'accepting' means one lot of letters goes inside another.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'tally' placed around 'h' is 'thally'.
'le'+'thally'='LETHALLY'
(Other definitions for lethally that I've seen before include "in fatal fashion" , "with deadly consequences" .)