Spectral maiden on horse in Xmas show (7)

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

M(abbrev. for “maiden”, in cricket scores) placed after ( on ) [ H(abbrev. for for “heroin” or “horse” in drug slang) contained in ( in ) PANTO(short for “pantomime”, a show traditionally performed at Xmas) ].