Clue: Dog Day Afternoon impresses one note Pacino's in the lead (8)
Answer: ALSATIAN
Explanation: Definition: Dog
 { SAT(abbrev. for "Saturday") + A(abbrev. for "afternoon") **contained in**(impresses) [ I(Roman numeral for "one") + N(abbrev. for "note") ] } **placed after**(…'s in the lead) AL(Pacino, American actor).
Defn: .. aka the German Shepherd.