I’m set to apprehend a bad lot initially allowing no opposition (10)
I believe the answer is:
implacable
'allowing no opposition' is the definition.
'i'm set to apprehend a bad lot initially' is the wordplay.
'set' becomes 'place' (placing is a kind of setting**).
'to apprehend' means one lot of letters goes inside another.
'initially' says to take the initial letters.
The initial letters of 'bad lot' is 'bl'.
'a'+'bl'='abl'
'place' enclosing 'abl' is 'placable'.
'im'+'placable'='IMPLACABLE'
(Other definitions for implacable that I've seen before include "Unable to be appeased" , "such as can't be appeased" , "unable to be stopped" , "Unwilling to be appeased" , "Totally unrelenting" .)