Clue: Spectral maiden on horse in Xmas show (7)
Answer: PHANTOM
Explanation: Definition: Spectral
 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) ].