"Filet" fish (4)
I believe the answer is:
sole
'filet fish' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sole that I've seen before include "Just one - fish" , "piece of leather" , "One and only; part of foot" , "Fish; part of foot" , "O __ Mio (Neapolitan song)" .)