Measuring instrument's forte, subtract one (5)
I believe the answer is:
meter
'measuring instrument's' is the definition.
(meter is a kind of measuring instrument)
'forte subtract one' is the wordplay.
I cannot really see how this works, but
'one' could be 'me' (a setter might use 'one' to humourously mean 'me') and 'me' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for meter that I've seen before include "Device that measures, say, electricity usage" , "Device for measuring and recording" , "Measuring device as for electricity consumption" , "Measuring device, say for electricity usage" , "Measuring device as for electricity usage (5)" .)