Be out of one’s depth identifying fish (8)
I believe the answer is:
flounder
I believe this is a double definition.
'be out of one's depth' is the first definition.
(I've seen this before)
'fish' is the second definition.
(flounder is a kind of fish)
'identifying' is the link.
(Other definitions for flounder that I've seen before include "Struggle clumsily - flatfish" , "thrash around wildly" , "Have difficulty" , "Be out of one's depth" , "Have difficulties with the flatfish" .)