There’s only one thing for it
Definition 1 of 1
Pro Tip 1/3Problem Solving
Used when you decide on a direct solution to a problem when no other options seem viable. 
When the printer wasn't working, there was only one thing for it – handwrite the notes.
Pro Tip 2/3Slight Urgency
Implies a need to act promptly because other alternatives might not be practical or possible. 
We're going to miss the show! There's only one thing for it – call a taxi now.
Pro Tip 3/3Decision Finality
Indicates that a final decision has been made after considering the situation. 
If the leak doesn't stop, there's only one thing for it – shut off the water supply.