Calibration seen on fish (5)
I believe the answer is:
scale
'calibration' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for scale that I've seen before include "Deposit formed in a kettle or on teeth" , "Climb; flake of skin" , "Measure" , "plaque" , "An ordered series of units for measurement purposes" .)