The mark Jack has to achieve (6)
I believe the answer is:
target
'the mark' is the definition.
('target' can be a synonym of 'mark')
'jack has to achieve' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'has' is a charade indicator (letters next to each other).
'to achieve' becomes 'get' (I've seen this in another clue).
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "Aim, ambition" , "bull's eye" , "Aim of an attack" , "aunt Sally" , "A result to aim at" .)