Letters on an ambulance (3)
I believe the answer is:
ems
'letters on an ambulance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ems that I've seen before include "Printers' measures" , "Printer's units, each two ens" , "German town" , "German river" , "spaces at the printing house" .)