Here we go -

>! a root — *i*   ($i=\sqrt{-1}$)  
>! a symbol — *pi*  
>! a division — *rip*  
>! a landing — *pier*  
>! dullness — *tripe*  
>! brightness — *esprit*  
>! a counter — *riposte*  
>! a liar — *imposter*  
>! a lie — *misreport*  
>! commanders — *imperators*  
>! substitutes — *temporaries*  
>! instruments — *polarimeters* or *vaporimeters*