Jack has to hit the mark (6)
I believe the answer is:
target
'the mark' is the definition.
('target' can be a synonym of 'mark')
'jack has to hit' is the wordplay.
'jack' becomes 'tar' (I've seen this before).
'has' is a charade indicator (letters next to each other).
'to hit' becomes 'get' (getting is a kind of hitting**).
'tar'+'get'='TARGET'
(Other definitions for target that I've seen before include "Object of attack" , "Point to be aimed at" , "A result to aim at" , "Something to aim for" , "Goal, objective" .)