Deception hard to prove (7)
I believe the answer is:
confirm
'prove' is the definition.
(I know that prove has the is a more specific form of the action confirm)
'deception hard' is the wordplay.
'deception' becomes 'con' (a con is a trick or deception).
'hard' becomes 'firm' (similar in meaning).
'con'+'firm'='CONFIRM'
'to' acts as a link.
(Other definitions for confirm that I've seen before include "Ratify; establish" , "Substantiate" , "Make (provisional arrangement) definite" , "Provide supporting evidence, make definite" , "Corroborate" .)