Some rules Pythagoras had to observe (4)
I believe the answer is:
espy
'observe' is the definition.
(thesaurus)
'some rules pythagoras had' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ESPY' can be found hidden inside 'rules pythagoras had'.
'to' acts as a link.
(Other definitions for espy that I've seen before include "Spot at a distance" , "Pick out at a distance" , "Spot; observe" , "Watch" , "Catch a glimpse of" .)