Drug given to fish? 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)
'drug given to fish?' is the wordplay.
'drug' becomes 'e' (e is another name for ecstasy).
'given to' means one lot of letters go next to another.
'fish?' becomes 'cod' (cod is a kind of fish**).
'e' after 'cod' is 'CODE'.
'there's' is the link.
(Other definitions for code that I've seen before include "Method of secret communication" , "Ethical standards" , "Cryptogram or set of standards" , "what's character-changing?" , "Secret symbols" .)