In what's written, crossword setter's reduced complicated devices (5)
I believe the answer is:
mazes
'complicated devices' is the definition.
(maze can mean something complex)
'in what's written crossword setter's reduced' is the wordplay.
'in' means one lot of letters goes inside another.
'what's written' becomes 'MS' (abbreviation for manuscript).
'crossword setter' becomes 'Azed' (Observer crossword setter).
'reduced' means to remove the last letter.
'azed' with its final letter taken away is 'aze'.
'ms' going around 'aze' is 'MAZES'.
(Another definition for mazes that I've seen is " Labyrinths".)