How to remove dullness of mind, without sitting meditation, in regular activity? When you have to perform a certain task, you can't meditate during that time, but mind needs clarity. I feel very dull during that time, I want to increase my awareness and alertness in that performance time.

So, how I can remove dullness of mind (while performing a certain task during which I can't meditate)? What are the causes that lead to dullness?
You can indeed meditate(Vipassana) while performing any task. 
ex: if you are sweeping, keep reminding: sweeping.. sweeping..  sweeping..

Dullness or boredom is a form of aversion. Ignorance is the cause of it. 
To overcome dullness, simply note it as it is without taking it as "I am bored" or "it is boring".