Deny policeman's allegation (8)
I believe the answer is:
disclaim
'deny' is the definition.
(I know that disclaim is a kind of deny)
'policeman's allegation' is the wordplay.
'policeman's' becomes 'di's' (abbreviation for Detective Inspector).
'allegation' becomes 'claim' (I've seen this in another clue).
'dis'+'claim'='DISCLAIM'
(Other definitions for disclaim that I've seen before include "Forswear" , "Renounce" , "Disown or deny, maybe responsibility" , "Deny, refuse to acknowledge" , "Deny or relinquish" .)