Order fish provided by day's end (6)
I believe the answer is:
codify
'order' is the definition.
(thesaurus)
'fish provided by day's end' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'provided' becomes 'if'.
'by' means one lot of letters go next to another.
'end' says to take the final letters.
The final letter of 'day' is 'y'.
'cod'+'if'+'y'='CODIFY'
(Other definitions for codify that I've seen before include "Collect and classify" , "Order" , "Arrange laws or rules into set" , "Organize into a system" , "Organise systematically" .)