Swimmer's extremely excitable, approaching lake (3)
I believe the answer is:
Here is my best explanation:
'swimmer's' is the definition.
(eel is a kind of fish)
'extremely excitable approaching lake' is the subsidiary indication.
'extremely' says to hollow out the word (remove centre letters).
'approaching' means one lot of letters go next to another.
'lake' becomes 'l'.
'excitable' with its middle removed is 'ee'.
(Other definitions for eel that I've seen before include "Fish in the Lee?", "A slippery character", "Evasive type", "Apode", "Slippery swimmer (may be jellied)", "Snakelike fish", "Creature like conger or moray", "Snaky fish", "User of electric organ" and "Slipper".)