have a trick up (one's) sleeve
To have a secret plan, idea, or advantage that can be utilized if and when it is required. A reference to cheating at a card game by hiding a favorable card up one's sleeve. I've got a few tricks up my sleeve if he decides to pursue legal action against me.
have an ˌace/a ˌtrick up your ˈsleeve
(British English) (American English have an ˌace in the ˈhole) (informal) have an idea or plan which you keep secret and can use if you need to (especially in order to gain an advantage over somebody): They think they’ve won the contract but we’ve still got a couple of aces up our sleeve.