Fish sauce -- at last there's a set of rules (4)
I believe the answer is:
code
'a set of rules' is the definition.
(I've seen this before)
'fish sauce at last' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish**).
'at last' suggests the final letters.
The final letter of 'sauce' is 'e'.
'cod'+'e'='CODE'
'there's' acts as a link.
(Other definitions for code that I've seen before include "System of signals" , "(Computer) programming instructions" , "Cryptographic system" , "It may be impossible to read" , "Specially devised system of words, as by Morse" .)