To expand on others comments, the options are:
1. CLX as Class1 scanner and the ETC module as Class1 adapter. This is implicit or IO communication using UDP (and usually multicast, but could be unicast). If the limits meet your requirements, this might be the best (easiest) option. The ETC supports three separate slave instance sets at around 500 bytes each (509 bytes T->O and 505 bytes O->T).
Since the CLX does not support Class1 slave adaptor functionality, it has to be CLX as scanner and ETC as adaptor.
2. Explicit messaging - use the CIP Modbus object and message from the CLX as client to the ETC device as server. In this way, you can read/write to any located variable (i.e. %MW1, etc) from the ETC.
3. Produce a tag on the CLX, and have the ETC consume it. If you add the eds file for the processor / Ethernet device in the ETC, you can actually enter in a produced tag name and read it using Class1 communications. However, this is limited as you can only consume data from the CLX so it is one-way communications.
4. Reverse engineer the native tag addressing protocol for CIP and use the SEND_REQUEST messaging block in the ETC to read a tag in the CLX device. As Yosi says, this is not for the faint of heart but it can work. RA has some guides that help on their website but you might need the ODVA specs, some reverse engineering from a known working driver on another product, and plenty of Wireshark to get it right.