Current measuring device encountered in more than half of America (7)
I believe the answer is:
ammeter
'current measuring device' is the definition.
(I've seen this before)
'encountered in more than half of america' is the wordplay.
I cannot quite understand how this works, but
'encountered' could be 'met' ('meet' can be a synonym of 'encounter') and 'met' is found in the answer.
'more' could be 'm' and 'm' is present in the answer.
'america' could be 'a' (common abbreviation - e.g. in organisation names) and 'a' is located in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ammeter that I've seen before include "Measurer of current flow" , "Electric current measurer" , "Instrument that measures electric current." , "Instrument to measure electric current" , "Instrument for measuring electric current" .)