Mickey has nothing to think about (5)
I believe the answer is:
mouse
'mickey' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'has nothing to think about' is the wordplay.
'has nothing' becomes 'o' (I've seen this in other clues).
'to think' becomes 'muse' (musing is a kind of thinking).
'about' indicates putting letters inside.
'o' inserted within 'muse' is 'MOUSE'.
Can you help me to learn more?
(Other definitions for mouse that I've seen before include "underhand office worker?" , "Quiet type" , "Timid creature" , "Tiny creature" , "See 1 down" .)