A hardened hitman is haunted by the ghost of one of his targets, an undercover cop, who compels him to hunt down the men who doubled-crossed them both and kidnapped the deceased cop’s estranged daughter.

I quite like the idea here, but a lot depends on the tone - I'd hope it doesn't take itself too seriously, but this is quite a 'heavy' logline.

I also think 'hunt down' and 'kidnapped' are covering the same ground - the cop basically wants his daughter rescued, so you could maybe tighten it along these lines:

"A hardened hitman is haunted by the ghost of one of his targets, an undercover cop, who needs the hitman's help to rescue his daughter from the men who doubled-crossed them both."