Optical illusion: lion disappears with 5 initially angry bats (6)

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

anagram/"bats" of ( i l u s V a )* i l u s letters are from "illusion", with the letters from "lion" removed/disappearing V is "5" in Roman numerals a is the initial of a-[ngry]