They may be under your feet when shifting fish (6)
I believe the answer is:
skates
I believe this is a double definition.
'they may be under your feet when shifting' is the first definition.
(eg roller skates)
'fish' is the second definition.
(skate is a type of fish)
(Other definitions for skates that I've seen before include "Fish, moves on ice" , "Boots for ice rink" , "You get them on to move fast" , "One glides on them, as over ice" , "Roller......,ice....." .)