This is the replacement spool holder on Daiwa SV spools, in white. It is a cam at the rotor base, to push out roter when it is twisted by magnetic force. Its older version in black tend to get stuck with rotor, or to break in time.
This white one is reinforced parts to replace the black spool holder.
The C clip holding rotor may be removed by 2 flat head scerw drivers. You'll surely need a tweezers to set it back, to hold a washer in position and to set the C clip in the groove. C clip and a washer are pushed by a spring, and they jump out. Please take care.