1) Hidden advantage 2) Hidden asset 3) Hidden resource 4) Last resort 5) Resource in reserve 6) Resource you might save 7) Secret weapon 
Ace in the hole is American slang for a final surprise which will likely guarantee success.
American slang for a final surprise which will likely guarantee success.
an ace dealt and held face down, esp. in stud poker. · an advantage or a resource kept back until the proper opportunity presents itself: His ace in the hole is his political influence.