Lack of fireworks in act sadly left one old lady cross (10)
I believe the answer is:
anticlimax
'lack of fireworks' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'in act sadly left one old lady cross' is the wordplay.
'sadly' is an anagram indicator.
'left one old lady cross' becomes 'limax' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'act'='inact'
'inact' is an anagram of 'antic'.
'antic'+'limax'='ANTICLIMAX'
Can you help me to learn more?
(Other definitions for anticlimax that I've seen before include "Let-down after excitement" , "disappointment" , "Come-down" , "Disappointing result" , "Damp squib" .)