Study boring degenerate animal (6)
I believe the answer is:
rodent
'animal' is the definition.
(rodent is a kind of animal)
'study boring degenerate' is the wordplay.
'study' becomes 'den' (term for a study or office).
'boring' indicates putting letters inside (some letters must 'bore' a hole into other letters).
'degenerate' becomes 'rot' (rotting is a kind of degenerating).
'den' placed within 'rot' is 'RODENT'.
(Other definitions for rodent that I've seen before include "Animal such as a mouse" , "Toothy mammal" , "Vermin" , "Eg, capybara" , "A rat, for instance" .)