Attest to sailor being involved in racket (8)
I believe the answer is:
notarise
'attest' is the definition.
(I know that notarise is a kind of attest)
'sailor being involved in racket' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'being involved in' indicates putting letters inside.
'racket' becomes 'noise' (racket can mean a loud noise).
'tar' inserted inside 'noise' is 'NOTARISE'.
'to' is the link.
(Other definitions for notarise that I've seen before include "Officially attest" , "establish an official record" , "Establish truth" , "Witness official documents" , "Certify (document) officially" .)