A set of rules about one German analgesic
I believe the answer is:
codeine
'analgesic' is the definition.
(codeine is a kind of analgesic)
'a set of rules about one german' is the wordplay.
'a set of rules' becomes 'code' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'one german' becomes 'ein' ('one' in German).
'code' enclosing 'ein' is 'CODEINE'.
(Other definitions for codeine that I've seen before include "Common opium-based tablet used to relieve pain" , "Derivative of opium used as an analgesic" , "Niece, do take this drug" , "Pain-relieving derivative of opium" , "Analgesic drug" .)