Cab fare calculator (5)
I believe the answer is:
meter
'cab fare calculator' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for meter that I've seen before include "Instrument for measuring quantity" , "Measuring device as for electricity usage (5)" , "Something measuring" , "Measuring device as for electricity consumption" , "Measuring device, say for electricity usage" .)