No servant recalled duke, a very special chap (7)
I believe the answer is:
'a very special chap' is the definition.
(diamond can mean a great person)
'no servant recalled duke' is the wordplay.
'servant' becomes 'maid' (maid is a kind of servant).
'recalled' says the letters should be written backwards.
'duke' becomes 'd' (abbreviation).
'nomaid' backwards is 'diamon'.
(Other definitions for diamond that I've seen before include "shape" , "Very hard gem" , "American field" , "Birthstone for April" , "(Usually colourless) gemstone - card" .)