Head of Police negligent? That's the assumption (7)
I believe the answer is:
premiss
'the assumption' is the definition.
('premiss' can be a synonym of 'assumption')
'head of police negligent?' is the wordplay.
'head of' suggests taking the first letters.
'negligent?' becomes 'remiss' ('remiss' can be similar in meaning to 'negligent').
The first letter of 'police' is 'p'.
'p'+'remiss'='PREMISS'
'that's' acts as a link.
(Other definitions for premiss that I've seen before include "Proposition" , "It's assumed" , "piece of logic" , "A condition stipulated beforehand" , "Assumption" .)