...below embankments: mischievous swimmers! (10)
I believe the answer is:
damselfish
'below embankments mischievous swimmers' is the definition.
I can't judge whether this definition defines the answer.
'below embankments mischievous' is the wordplay.
'below' means one lot of letters go next to another (in a down clue, some letters go below others).
'embankments' becomes 'dams' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mischievous' becomes 'elfish' (synonyms).
'dams'+'elfish' is 'DAMSELFISH'.
Can you help me to learn more?
(Another definition for damselfish that I've seen is " Swimmer".)