Swindle hard to prove (7)
I believe the answer is:
confirm
'prove' is the definition.
(thesaurus)
'swindle hard' is the wordplay.
'swindle' becomes 'con' (synonyms).
'hard' becomes 'firm' (similar in meaning).
'con'+'firm'='CONFIRM'
'to' is the link.
(Other definitions for confirm that I've seen before include "Establish the truth of" , "Ratify; establish" , "Authenticate" , "Ratify or verify" , "Substantiate" .)