Think about nothing but a rodent (5)
I believe the answer is:
mouse
'a rodent' is the definition.
(I know that mouse is a type of rodent)
'think about nothing' is the wordplay.
'think' becomes 'muse' (to muse on something is to think about it).
'about' means one lot of letters goes inside another.
'nothing' becomes 'O' (looks like zero - 0).
'muse' going around 'o' is 'MOUSE'.
'but' is the link.
(Other definitions for mouse that I've seen before include "Quiet type" , "computer equipment" , "Timid fellow" , "whose movements are captured on screen" , "Cursor control" .)