Shakes tail to detach from one shelled seafood item (4,6)
I believe the answer is:
rock salmon
'one shelled seafood item' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'shakes tail to detach from' is the wordplay.
'shakes' becomes 'rocks' ('rock' can be a synonym of 'shake').
'tail' means to remove the last letter (I've seen 'tailed' mean this (to tail something can mean to remove its end)).
'to detach from' becomes 'salmon' (I am not sure about this - if you are sure you should believe this answer much more).
'rocks' with its last letter taken off is 'rock'.
'rock'+'salmon'='ROCK-SALMON'
Can you help me to learn more?
(Other definitions for rock salmon that I've seen before include "Catfish as food" , "Dogfish as food" , "swimmer" , "Fish" .)