Rude cashier's opening safe (9)
I believe the answer is:
charmless
'rude' is the definition.
Both the answer and definition are adjectives.
Perhaps they are linked in a way I don't understand?
'cashier's opening safe' is the wordplay.
'opening' suggests taking the first letters.
'safe' becomes 'harmless' (similar in meaning).
The first letter of 'cashiers' is 'c'.
'c'+'harmless'='CHARMLESS'
Can you help me to learn more?
(Other definitions for charmless that I've seen before include "Unattractive" , "Without attractive features" , "not winning" , "Lacking any appeal" .)