Macro clear_history 

macro_rules! clear_history {
    (@cmd ($cmd:expr) -t $target_pane:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -t $target_window:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -p $pane_index:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr)) => { ... };
    () => { ... };
    (($cmd:expr), $($tail:tt)*) => { ... };
    ($($tail:tt)*) => { ... };
}

Remove and free the history for the specified pane.

Manual

tmux ^1.0:

clear-history [-t target-pane]
(alias: clearhist)

tmux ^0.9:

clear-history [-p pane-index] [-t target-window]
(alias: clearhist)