Fish beneath small rock (5)
I believe the answer is:
shake
'rock' is the definition.
(I know that rock can be written as shake)
'fish beneath small' is the wordplay.
I cannot quite see how this works, but
'fish' could be 'hake' (type of fish) and 'hake' is present in the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for shake that I've seen before include "wave" , "Flavoured milk drink" , "rock" , "Rattle" , "Tremble, vibrate" .)