Proserpine, frantic with lust, beds a group containing Mars? (8,7)

'proserpine frantic with lust beds' is the definition.
I can't tell whether this definition defines the answer.

'a group containing mars?' is the wordplay.
I cannot really see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found in the answer.
'mars?' could be 'planet' (mars is a kind of planet) and 'planet' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?