Complex number precedes zero (7)
I believe the answer is:
nothing
'zero' is the definition.
(synonyms)
'complex number precedes' is the wordplay.
'complex' becomes 'thing' (I am not sure about this - if you are sure you should believe this answer much more).
'number' becomes 'no' (abbreviation for 'number').
'precedes' means one lot of letters go next to another (some letters come before others).
'thing' after 'no' is 'NOTHING'.
Can you help me to learn more?
(Other definitions for nothing that I've seen before include "Something of no concern" , "'Zero, zilch (7)'" , "without object" , "can mean love" , "Duck" .)