Playground amenity was set up by diocese (6)
I believe the answer is:
seesaw
'playground amenity' is the definition.
'was set up by diocese' is the wordplay.
'set up' says the letters should be written backwards (in a down clue, letters go up).
'by' means one lot of letters go next to another.
'diocese' becomes 'see' (I've seen this before).
'was' backwards is 'saw'.
'saw' after 'see' is 'SEESAW'.
(Other definitions for seesaw that I've seen before include "Plaything that's ridden up and down" , "Children's playground item" , "pitch" , "Fluctuate" , "Vacillate; playground feature" .)