Very detailed, certain it's complicated (9)
I believe the answer is:
intricate
'very detailed' is the definition.
(I've seen this before)
'certain it's complicated' is the wordplay.
'complicated' indicates an anagram.
'certain'+'it'='certainit'
'certainit' with letters rearranged gives 'INTRICATE'.
(Other definitions for intricate that I've seen before include "Entangled" , "ornate" , "Convoluted" , "Fancy" , "Complicated, detailed" .)