Mata Hari, say, losing head over lover once, a person of some standing (9)
I believe the answer is:
gentleman
'a person of some standing' is the definition.
I can't tell whether this definition defines the answer.
'mata hari say losing head over lover' is the wordplay.
'mata hari say' becomes 'agent' (mata hari is a kind of agent).
'losing head' means to remove the first letter.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'lover' becomes 'leman' (I've seen this before).
'agent' with its first letter taken away is 'gent'.
'gent'+'leman'='GENTLEMAN'
'once' is the link.
Can you help me to learn more?
(Other definitions for gentleman that I've seen before include "trustworthy type" , "Polite, kind or upper-class fellow" , "Toff" , "-'s -; - farmer" , "Courteous chap" .)