Should find nothing missing, initially (5)
I believe the answer is:
ought
'should' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ought that I've seen before include "Tough (anag)" , "It expresses duty or obligation" , "Should, had, better" , "Verb implying duty; zero, nothing (archaic)" , "Anything at all (archaic)" .)