A patient collector (9)

I believe the answer is:

ambulance

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

(Other definitions for ambulance that I've seen before include "Vehicle for transporting the sick" , "Vehicle for sick or injured" , "Vehicle for carrying the sick" , "what's used by 9a 19d" , "Vehicle for transport to hospital" .)

I've seen this clue in The Globe and Mail.

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