Popular accountant stifled by banal complex (9)
I believe the answer is:
intricate
'complex' is the definition.
('intricate' can be similar in meaning to 'complex')
'popular accountant stifled by banal' is the wordplay.
'popular' becomes 'in'.
'accountant' becomes 'ca' (abbreviation for chartered accountant).
'stifled by' indicates putting letters inside (I've seen this in other clues).
'banal' becomes 'trite' (I've seen this before).
'ca' inserted within 'trite' is 'tricate'.
'in'+'tricate'='INTRICATE'
(Other definitions for intricate that I've seen before include "ornate" , "puzzlingly obscure" , "Complicated, detailed" , "Convoluted" , "Fancy" .)