Evelyn half concealed unknown wild cat (4)
I believe the answer is:
lynx
'wild cat' is the definition.
(wild cat species)
'evelyn half concealed unknown' is the wordplay.
'half concealed' indicates taking half.
'unknown' becomes 'x' (algebra).
'evelyn' with half the letters taken is 'lyn'.
'lyn'+'x'='LYNX'
(Other definitions for lynx that I've seen before include "One with sharp eyes" , "one with exceptionally good sight" , "Cat - northern constellation" , "Big cat" , "Wild cat with yellowish-brown fur and tufted ears" .)