Fish under small rock (5)
I believe the answer is:
shake
'rock' is the definition.
(I know that rock can be written as shake)
'fish under small' is the wordplay.
I cannot quite understand how this works, but
'fish' could be 'hake' (type of fish) and 'hake' is found within the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in 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 "Flavoured milk drink" , "moment" , "Shiver" , "Tremble; milk drink" , "Judder" .)