'a single tear' is correct and usable in written English.
You can use it to express emotion, particularly sadness or grief. For example, "She shed a single tear as she said goodbye."
And then a single tear rolls down his bear cheek.
I shed a single tear for those shoes.
A single tear rolled down Darnell's cheek a little before noon.
As she winced, Nakal allowed only a single tear to fall down her cheek.
Indeed, handing it over to Mr. Davenport, I believe I shed a single tear.
Early on, Mr. Davenport drew the challenge "You have two minutes to produce a single tear".
This week's image of President Lincoln with a single tear was illustrated by Tim O'Brien, who also created the iconic Time cover of President Reagan with a single tear.
As he watches the last scene of "Public Enemy," stony faced, he actually chokes back a single tear.
For several hours they did not fire anything except a single tear gas canister, which was thrown back at them.
Because she knew she had cried, and yet she had no memory of a single tear falling.
