It measures current in the morning and energy used in new term (7)
'it measures current' is the definition.
(I've seen this before)
'morning and energy used in new term' is the wordplay.
'morning' becomes 'am'.
'and' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'used in' is an insertion indicator.
'new' is an anagram indicator.
'term' with letters rearranged gives 'metr'.
'e' put within 'metr' is 'meter'.
'in the' acts as a link.
(Other definitions for ammeter that I've seen before include "Instrument measuring electrical current", "Instrument that measures electric current.", "Current-measurer", "Device that measures the flow of electrical current", "Device measuring electric current".)