Think there's nothing inside but there is an animal (5)
I believe the answer is:
mouse
'there is an animal' is the definition.
(mouse is a kind of animal)
'think there's nothing inside' is the wordplay.
'think' becomes 'muse' (to muse on something is to think about it).
'there's nothing' becomes 'o' (I've seen this in other clues).
'inside' is an insertion indicator.
'muse' going around 'o' is 'MOUSE'.
'but' is the link.
(Other definitions for mouse that I've seen before include "Small rodent - computer attachment" , "shy and timid type" , "PC device" , "retiring person" , "control of computer system" .)