The first fish dish (4)
I believe the answer is:
tray
'dish' is the definition.
(thesaurus)
'the first fish' is the wordplay.
'the first' becomes 't' (rst letter of 'the').
'fish' becomes 'ray' (ray is a kind of fish).
't'+'ray'='TRAY'
(Other definitions for tray that I've seen before include "three" , "Portable carrier" , "Flat plate for carrying things" , "Carrying container" , "Correspondence container" .)