Fish served in ornate trattoria (5)
I believe the answer is:
tetra
'fish' is the definition.
(tetra is a kind of fish)
'in ornate trattoria' is the wordplay.
'in' indicates a hidden word.
'TETRA' is hidden in the letters of 'ornate trattoria'.
'served' acts as a link.
(Other definitions for tetra that I've seen before include "Denoting four (prefix)" , "Swimmer" , "This indicates a group of four" , "rarely seen flower" , "... Four < , in composition" .)