Characteristic study I confess primarily into catlike animals (8)
I believe the answer is:
genetics
'characteristic study' is the definition.
(study of genetic characteristics)
'i confess primarily into catlike animals' is the wordplay.
'primarily' indicates taking the first letters.
'into' means one lot of letters goes inside another.
'catlike animals' becomes 'genets' (genets are catlike mammals).
The initial letter of 'confess' is 'c'.
'i'+'c'='ic'
'ic' going inside 'genets' is 'GENETICS'.
(Other definitions for genetics that I've seen before include "The scientific study of heredity" , "Science of heredity" , "Branch of biology" , "Study of inherited characteristics" , "The study of biological heredity" .)