Even flipping 50%, initially switching heads and tails (4)
I believe the answer is:
flat
'even' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for flat that I've seen before include "with no relief" , "like some races" , "Puncture" , "Level - uninteresting - accommodation" , "unexciting" .)