Sister protects top of head from devilish creature swimming in the sea (6)
I believe the answer is:
shrimp
'swimming in the sea' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'sister protects top of head from devilish creature' is the wordplay.
'sister' becomes 'sr' (I've seen this in other clues).
'protects' means one lot of letters goes inside another.
'top of' indicates taking the first letters.
'from' says to put letters next to each other.
'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 initial letter of 'head' is 'h'.
'sr' enclosing 'h' is 'shr'.
'shr'+'imp'='SHRIMP'
Can you help me to learn more?
(Other definitions for shrimp that I've seen before include "Small shellfish or very small person" , "tiny chap" , "Tot" , "ingredient of cocktail" , "Diminutive person" .)