Ancient money card rejected by Her Majestic (6)
I believe the answer is:
drachm
'ancient money' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'card rejected by her majestic' is the wordplay.
I cannot really see how this works, but
an anagram of 'card' is 'drac' which is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for drachm that I've seen before include "Old Greek weight" , "One eighth of a fluid ounce" , "Apothecaries' unit of weight" , "Old liquid measure" , "Fluid measure" .)