Dinner's starter, wonderful fish (4)
I believe the answer is:
dace
'fish' is the definition.
(I've seen this before)
'dinner's starter wonderful' is the wordplay.
'starter' says to take the initial letters.
'wonderful' becomes 'ace'.
The initial letter of 'dinners' is 'd'.
'd'+'ace'='DACE'
(Other definitions for dace that I've seen before include "Small river fish" , "Fish in the carp family" , "Chubby swimmer" , "Freshwater fish" .)