Diana's a girl cashier (7)
I believe the answer is:
dismiss
'cashier' is the definition.
(I've seen this before)
'diana's a girl' is the wordplay.
'diana's' becomes 'Di's' (Di is short for Diana).
'a girl' becomes 'miss' (term of address for an unmarried woman).
'dis'+'miss'='DISMISS'
(Other definitions for dismiss that I've seen before include "Remove from office" , "Sack - send away" , "Reject, send away" , "military order" , "Drop" .)