Go up for a bit of fish (5)
I believe the answer is:
scale
'go up' is the definition.
(scaling is a kind of going up)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for scale that I've seen before include "Horny plate; balance" , "Notes < magnitude" , "Climb, or series of notes in music" , "Plate protecting the skin of fish and reptiles" , "Relative extent as of laces" .)