Only one of the denizens of the deep (4)
I believe the answer is:
sole
'only one' is the definition.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for sole that I've seen before include "Exclusive class of fish" , "Dover - -" , "Unique" , "Underside of the foot" , "O __ Mio (Neapolitan song)" .)