Old meter readout is radical (7)
I believe the answer is:
extreme
'is radical' is the definition.
(radical can mean revolutionary or extreme)
'old meter readout' is the wordplay.
I cannot quite see how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is located in the answer.
an anagram of 'meter' is 'treme' which is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for extreme that I've seen before include "Very intense - way-out" , "Exceptional - maximum - farthest" , "Severe; radical" , "end" , "Of the greatest intensity" .)