Lack of fireworks in act sadly left one old lady cross (10)

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

Anagram (sadly) of IN ACT , then L (abbreviation for left) + I (one in Roman numerals) + MA (old lady = slang for mother) + X (a cross). Anticlimax = lack of fireworks = something that isn’t as exciting as expected.