Diner's first to get fabulous fish (4)
I believe the answer is:
dace
'fish' is the definition.
(dace is a kind of fish)
'diner's first to get fabulous' is the wordplay.
'diner's first' becomes 'd' (1st letter of 'diner').
'to get' means one lot of letters go next to another.
'fabulous' becomes 'ace' (both can mean excellent).
'd'+'ace'='DACE'
(Other definitions for dace that I've seen before include "Small river fish" , "Chubby swimmer" , "Freshwater fish" , "Fish in the carp family" .)