Nothing taken, head off (5)
'nothing' is the definition.
(I've seen this before)
'taken head off' is the wordplay.
'taken' becomes 'bought' ('buy' can be a synonym of 'take').
'head off' means to remove the first letter.
'bought' with its first letter removed is 'OUGHT'.
(Other definitions for ought that I've seen before include "Has a duty (to do something)" , "outdated anything at all?" , "Strange thing" , "Be bound" , "Should, had, better" .)
'nothing' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for aught that I've seen before include "Nothing" , "Anything (Shakespearean)" , "Emphatically anything" , "Anything at all (archaic)" .)