Confirm Jack is part of racket (8)
I believe the answer is:
notarise
'confirm' is the definition.
('confirm'->'notarize' is in my internal thesaurus and 'ize'->'ise')
'jack is part of racket' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'is part of' is an insertion indicator (part of the inside).
'racket' becomes 'noise' (synonyms).
'tar' inserted into 'noise' is 'NOTARISE'.
(Other definitions for notarise that I've seen before include "Authenticate" , "establish an official record" , "Certify (document) officially" , "Officially attest" , "Establish truth" .)