Lift head up, getting first prize (7)
I believe the answer is:
Here is my best explanation:
'first prize' is the definition.
'lift head up' is the subsidiary indication.
'lift' becomes 'jack' (jacking is a kind of lifting).
'head' becomes 'top' ('top' can be a synonym of 'head').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'top' in reverse letter order is 'pot'.
'getting' is the link.
(Other definitions for jackpot that I've seen before include "Big payout", "first prize", "Great thing to hit?", "Cumulative prize", "Hit the . . . . . . . - win the big prize", "pool", "Large cash prize", "Accumulated prize", "The big pay-out - hit it" and "major win".)