Show that this income tax is effectively a lump sum tax
A lump sum tax is a tax which only has an income effect, it has the equivalent effect of reducing the agent's wealth. In particular, it has no substitution effect, it does not incentivise the agent to switch between $c$ and $l$.

In your case the agent's optimal choice of $c$ and $l$ is given by:
$(c^*,l^*)=\left(\frac{w(1-t)}{1+w(1-t)},\frac{w(1-t)}{1+w(1-t)}\right)$ 
(show this)

It is easy to see that increasing $t$ is equivalent to decreasing $w$.