Get rid of small fish, rotten on the underside (5,3)
I believe the answer is:
shake off
'get rid of' is the definition.
(I've seen this before)
'small fish rotten on the underside' is the wordplay.
I cannot really see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'fish' could be 'hake' (type of fish) and 'hake' is found within the answer.
'rotten' could be 'off' (rotten food is 'off') and 'off' 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 off that I've seen before include "Get away from (followers)" , "Get rid of" .)