Flag down
I believe the answer is:
hail
'flag down' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hail that I've seen before include "An old Roman salute as to Caesar" , "Old Roman salute, or frozen pellets" , "Welcome - precipitation" , "Greet - bad weather" , "Call from a distance" .)