Measuring instrument (5)
(Other definitions for meter that I've seen before include "Device that measures, say, electricity usage" , "Poe sometimes used this" , "Something measuring" , "Gauge" , "Device to measure quantity used of fluid, electricity etc" .)
'measuring instrument' is the definition.
(gauge is a kind of measuring instrument)
This is the entire clue.
(Other definitions for gauge that I've seen before include "Measure(r)" , "Measure against a standard" , "Measuring device" , "standard size" , "Estimate, form judgement of" .)