Jolly Jack seen aboard sturdy launch (5,3)
I believe the answer is:
start out
'launch' is the definition.
(I've seen this before)
'jolly jack seen aboard sturdy' is the wordplay.
'jolly jack' becomes 'tar' (Jolly Jack Tar can mean a sailor).
'seen aboard' is an insertion indicator.
'sturdy' becomes 'stout' (synonyms).
'tar' placed inside 'stout' is 'START OUT'.
(Other definitions for start out that I've seen before include "Embark (on a venture)" , "Begin a journey" , "Go off" , "launch" , "Set off" .)