Deny policeman broadcast to catch a vandal principally (7)
I believe the answer is:
disavow
'deny' is the definition.
(disavowing is a kind of denying)
'policeman broadcast to catch a vandal principally' is the wordplay.
'policeman' becomes 'di' (I've seen this in other clues).
'broadcast' becomes 'sow' (sowing is a kind of broadcasting).
'to catch' means one lot of letters goes inside another.
'a vandal principally' becomes 'av' (I can't justify this - if you can you should give a lot more credence to this answer).
'sow' enclosing 'av' is 'savow'.
'di'+'savow'='DISAVOW'
Can you help me to learn more?
(Other definitions for disavow that I've seen before include "Deny responsibility for (something)" , "To deny support" , "To deny knowledge of" , "Disclaim" , "Refuse to acknowledge" .)