Should, had better (5)
I believe the answer is:
ought
I believe this clue is a double definition.
'should' is the first definition.
(synonyms)
'had better' is the second definition.
(synonyms)
(Other definitions for ought that I've seen before include "Anything at all (archaic)" , "Verb implying duty; zero, nothing (archaic)" , "outdated anything at all?" , "It expresses duty or obligation" , "Be logically necessary" .)