Riddle with complex meaning any number missed (6)
I believe the answer is:
enigma
'riddle' is the definition.
(I know that riddle can be written as enigma)
'complex meaning any number missed' is the wordplay.
'complex' is an anagram indicator.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'missed' indicates named letters should be taken away.
'meaning' with 'n' taken out is 'meaing'.
'meaing' with letters rearranged gives 'ENIGMA'.
'with' is the link.
(Other definitions for enigma that I've seen before include "It's mysterious, difficult to understand" , "Mysteriously puzzling person or thing" , "Mysterious puzzle" , "Coding device" , "Riddle - German coding machine" .)