Disturbance involving Jolly Jack — establish truth? (8)
I believe the answer is:
notarise
'establish truth?' is the definition.
(to notarise is to create a truthful record)
'disturbance involving jolly jack' is the wordplay.
'disturbance' becomes 'noise' (synonyms).
'involving' means one lot of letters goes inside another.
'jolly jack' becomes 'tar' (Jolly Jack Tar can mean a sailor).
'noise' enclosing 'tar' is 'NOTARISE'.
(Other definitions for notarise that I've seen before include "Officially attest" , "Authenticate" , "Establish truth" , "Witness official documents" , "establish an official record" .)