Objective for Jack to achieve (6)
I believe the answer is:
target
'objective' is the definition.
(I know that objective can be written as target)
'jack to achieve' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'to achieve' becomes 'get' (I've seen this in another clue).
'tar'+'get'='TARGET'
'for' acts as a link.
(Other definitions for target that I've seen before include "Objective, mark" , "Point to aim at (6)." , "Aim, objective" , "Aim, ambition" , "Goal to be aimed for" .)