Revolutionary period involved in no end of vile execution by drowning (6)
I believe the answer is:
noyade
'execution by drowning' is the definition.
(I've seen this before)
'revolutionary period involved in no end of vile' is the wordplay.
'revolutionary' says the letters should be written backwards.
'period' becomes 'day' (day is a kind of period).
'involved in' means one lot of letters goes inside another.
'end of' suggests the final letters.
The final letter of 'vile' is 'e'.
'day' in reverse letter order is 'yad'.
'no'+'e'='noe'
'yad' inserted into 'noe' is 'NOYADE'.
(Other definitions for noyade that I've seen before include "despatched by drowning" , "execution at Nantes" , "Execution by drowning" , "Form of massacre" .)