Old-fashioned and rejected - like a diamond (6,3)
I believe the answer is:
square cut
'a diamond' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'old-fashioned and rejected like' is the wordplay.
'old fashioned' becomes 'square'.
'and' is a charade indicator (letters next to each other).
'rejected' says the letters should be written backwards.
'like' becomes 'tuc' (I can't explain this - if you can you should believe this answer much more).
'tuc' written backwards gives 'cut'.
'square'+'cut'='SQUARE-CUT'
Can you help me to learn more?
(Other definitions for square cut that I've seen before include "Shot" , "having a strong chin?" , "three instead of four?" , "producing bang?" , "so play this?" .)