What do 肯定 and 一定 do in 这些行动是积极的，值得肯定，对于减少烟毒有一定的意义?
The whole sentence is : 这些行动是积极的(These actions are positive), 值得肯定(worthy of recognition),对于减少烟毒有一定的意义。(have certain significance to reduce the smoke toxicity).

Here 肯定 means recognition by people or society.比如 他的事迹得到了社会的肯定。
There is no 一定的意 in Chinese
一定的意义 means certain significance.比如 你的建议对我有一定的价值，我会好好想想。(Your suggestions has some certain value for me, and I will think over it.)