It's essential to calibrate this device (6)
I believe the answer is:
scales
'essential to calibrate this device' is the definition.
Although both the answer and 'device' are nouns, they are not in the same form of the noun.
'it's' is the wordplay.
I cannot really understand how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
This explanation may well be incorrect...
(Other definitions for scales that I've seen before include "Graduated sequence of musical notes" , "Climbs the weight measuring device" , "Libra, Sign of the Zodiac" , "Weighing devices or fish flakes" , "Sequences of musical notes" .)