Cab signal (4)
I believe the answer is:
hail
'cab signal' 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 "Precipitation - old greeting" , "Call from a distance" , "Greeting - falling stones" , "a drop of the hard stuff?" , "Greet enthusiastically" .)