Old person and resistance spies abducted by Martians allegedly (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

(R{esistance} CIA(=spies)) in men(=Martians). The latter is a reference to the 1992 John Gray book, "Men Are from Mars, Women Are from Venus". Mercia was an old kingdom of England, so someone living there would be an "old person" in the sense of a dweller in ancient times.