I don't have a problem with "a still significant", particularly if the context includes some comment about diminishing numbers of smokers. The sentence looks fine to me. It's a bit more "academic" in tone than straightforward, but it's still very clear.
I found these examples of "a still significant" via Google:
"Canada's productivity performance has been disappointing in recent years, with estimates suggesting a still significant gap with the level in the United States."
"....and a still significant gap between groups of countries with prevalence rates of more than 50%..."
"Other recommended provisions simply do not reduce a still significant risk."
"...that a still significant degree of economic slack remains in Japan..."