Notice ends of Möbius strip rarely (3)
I believe the answer is:
spy
'notice' is the definition.
(spy can mean to notice or observe)
'ends of mobius strip rarely' is the wordplay.
'ends of' indicates one should take the final letters (I've seen 'end of' mean this).
The last letters of 'mobius strip rarely' are 'SPY'.
(Other definitions for spy that I've seen before include "Undercover agent" , "Watch covertly like secret agent" , "Enemy agent" , "Keep watch secretly, maybe for foreign government" , "Secret agent of government" .)