Online algorithm for finding of clique of size k
Since I don't have enough points for a comment:

This problem is not going to have a very efficient solution. Looking at the $k=3$ case should be good start. In this case one can solve this problem with $O(|E|)$ space by keeping an (online) adjacency set for each vertex and when an edge $(u,v)$ arrives comparing the adjacency sets of vertices $u$ and $v$ for a collision (in time linear in the size of the smaller set) and then adding $(u,v)$ to the sets of vertex $u$ and $v$.

The run time of this algorithm would be $\sum_v d(v)^2$, where $d(v)$ is the degree of vertex $v$ in the final graph. This is never bigger than $O(n^3)$, where $n$ is the number of vertices in the final graph.

It is unknown whether triangle detection (i.e., a clique of size 3) can be done in  less than $n^3$ time even if you had the entire graph before you. See this, for instance.