Complex to explain (9)
I believe the answer is:
elaborate
This clue is a double definition.
'complex' is the first definition.
(thesaurus)
'explain' is the second definition.
(thesaurus)
'to' is the link.
(Other definitions for elaborate that I've seen before include "Fussy" , "with added detail" , "Complicated - work out" , "Intricate - ornament" , "Highly detailed" .)