Being above the lower forms of life, they have no tails (5)
I believe the answer is:
heads
'tails' is the definition.
The answer and definition can be both animals as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for heads that I've seen before include "Leaders" , "a 50-50 chance" , "Obverse result of coin-tossing" , "Noddles; nuts" , "Chiefs" .)