Unlikely to happen as principal leaves hurriedly (4)

I believe the answer is:

long

I'm a little stuck... Click here to teach me more about this clue!

But I can't explain it at all! Could you help me understand?

(Other definitions for long that I've seen before include "Protracted" , "to be tall" , "summer break?" , "Yearn (for)" , "Pant" .)

Want a hint initially instead of a full solution? Install my app