Rocks passed in the middle, concealing fish (6)
I believe the answer is:
shakes
'rocks' is the definition.
('shake' can be a synonym of 'rock')
'passed in the middle concealing fish' is the wordplay.
'in the middle' indicates the central letters.
'concealing' means one lot of letters goes inside another.
'fish' becomes 'hake' (hake is a kind of fish).
The middle of 'passed' is 'ss'.
'ss' going around 'hake' is 'SHAKES'.
(Other definitions for shakes that I've seen before include "Moves quickly up and down, say milk drink" , "jars" , "Frothy drink of milk and flavouring" , "Quivers" , "Judders" .)