Microcontroller as a slave
You can install Simple Open EtherCAT Slave or SOES 
on a microcontroller. SOES supports the Distributed Clock mode.

To estimate the effort of installing the SOES on a microcontroller,
have a look at https://bitbucket.org/utwente_bss/soes_arm/wiki/Home .
The slave version referred to in the wiki is not the latest but gives you enough hints.

Another option is light ethercat slave. This one is the most hardware independent.