Assumption made by head of police failing in duty (7)
I believe the answer is:
premiss
'assumption' is the definition.
(synonyms)
'head of police failing in duty' is the wordplay.
I cannot really understand how this works, but
'failing' could be 'remiss' (I've seen this before) and 'remiss' is found in the answer.
A single letter 'p' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'made by' is the link.
Can you help me to learn more?
(Other definitions for premiss that I've seen before include "Assumption" , "It's assumed" , "Proposition" , "A condition stipulated beforehand" , "piece of logic" .)