Measuring device agreed with every reading initially (5)
I believe the answer is:
meter
'measuring device' is the definition.
(I've seen this before)
'agreed with every reading initially' is the wordplay.
'agreed' becomes 'met' (meeting is a kind of agreeing**).
'with' means one lot of letters go next to another.
'initially' says to take the initial letters.
The initial letters of 'every reading' is 'er'.
'met'+'er'='METER'
(Other definitions for meter that I've seen before include "Gauge" , "Poe sometimes used this" , "Measuring device as for electricity usage (5)" , "Device to measure quantity used of fluid, electricity etc" , "Measuring device, say for electricity usage" .)