Swimmer's extremely excitable, approaching lake (3)
I believe the answer is:
eel
'swimmer's' is the definition.
(eel is a kind of aquatic vertebrate)
'extremely excitable approaching lake' is the wordplay.
'extremely' suggests removing the centre.
'approaching' says to put letters next to each other.
'lake' becomes 'l'.
'excitable' with its centre removed is 'ee'.
'ee'+'l'='EEL'
(Other definitions for eel that I've seen before include "Almost scaleless fish" , "Teleost fish" , "Evasive type" , "Electric swimmer, perhaps?" , "Fish in the Lee?" .)