Order piece of fish (5)
I believe the answer is:
scale
This might be a double definition.
'order' is the first definition.
(thesaurus)
'piece of fish' is the second definition.
The definition and answer can be both animals as well as being singular nouns.
I also know that 'fish' relates to this answer.
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" , "Graduated table" , "Climb, or series of notes in music" , "Relative size (of something)" , "Measure" .)