Rules you should follow? Endless medicine! (3)

I believe the answer is:

dos

'rules you should follow?' is the definition.
(as in dos and don'ts)

'endless medicine' is the wordplay.
'endless' means to remove the last letter.
'medicine' becomes 'dose' (dose is a kind of medicine).
'dose' with its final letter taken off is 'DOS'.

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.