Pluto to declare and deny (7)
I believe the answer is:
disavow
'deny' is the definition.
(I know that disavow is a type of deny)
'pluto to declare' is the wordplay.
'pluto' becomes 'dis' (another name for the mythical god).
'to declare' becomes 'avow' (avowing is a kind of declaring).
'dis'+'avow'='DISAVOW'
'and' is the link.
(Other definitions for disavow that I've seen before include "Refuse to acknowledge" , "To deny support" , "Deny responsibility for (something)" , "To deny knowledge of" , "Disclaim" .)