Maid turned on duke, one in red suit (7)
I believe the answer is:
diamond
'red suit' is the definition.
'maid turned on duke' is the wordplay.
'turned' says the letters should be written in reverse.
'duke' becomes 'd' (abbreviation).
'maid' reversed gives 'diam'.
'diam'+'on'+'d'='DIAMOND'
'one in' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for diamond that I've seen before include "suited one" , "Sixtieth" , "American field" , "Birthstone for April" , "Red card" .)