Expert identifying species of flatfish (3)
I believe the answer is:
dab
I believe this is a double definition.
'expert' is the first definition.
(as in a dab hand)
'species of flatfish' is the second definition.
(dab is a type of flatfish)
'identifying' is the link.
(Other definitions for dab that I've seen before include "Lightly press" , "Expert; flatfish" , "[SOMETHING FISHY]" , "A small fish - bad one" , "Flatfish; fingerprint" .)