Sister protects top of head from devilish creature swimming in the sea (6)
I believe the answer is:
Here is my best explanation:
'swimming in the sea' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'sister protects top of head from devilish creature' is the subsidiary indication.
'sister' becomes 'sr' (I've seen this in other clues).
'protects' indicates putting letters inside.
'top of' suggests taking the first letters.
'from' means one lot of letters go next to another.
'devilish creature' becomes 'imp' (I have seen 'Mischievous creature' mean 'imp' so perhaps 'creature' could also mean 'imp'. I am not sure about the 'devilish' bit.**).
The first letter of 'head' is 'h'.
'sr' placed around 'h' is 'shr'.
(Other definitions for shrimp that I've seen before include "Seafood", "Edible crustacean", "Tot", "Prawn-like creature", "who gets bullied?", "colour?", "ingredient of cocktail", "no giant", "School member" and "Small edible shellfish, pink when boiled".)