Strangely certain it is complicated (9)
I believe the answer is:
intricate
'is complicated' is the definition.
(thesaurus)
'strangely certain it' is the wordplay.
'strangely' is an anagram indicator.
'certain'+'it'='certainit'
'certainit' with letters rearranged gives 'INTRICATE'.
(Other definitions for intricate that I've seen before include "ornate" , "puzzling" , "Fancy" , "Complicated, detailed" , "Very complicated and involved" .)