Doubt detectives contradict sheriff, ultimately (9)
I believe the answer is:
disbelief
'doubt' is the definition.
(disbelief is a kind of doubt)
'detectives contradict sheriff ultimately' is the wordplay.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'contradict' becomes 'belie' ('belie' can be a synonym of 'contradict').
'ultimately' suggests the final letters.
The final letter of 'sheriff' is 'f'.
'dis'+'belie'+'f'='DISBELIEF'
(Other definitions for disbelief that I've seen before include "Refusal to advance credit" , "Incredulity" , "having exhausted trust fund?" , "Lack of faith" .)