Cat perhaps, one thinking about nothing (6)
I believe the answer is:
mouser
'cat perhaps' is the definition.
(I've seen this in another clue)
'one thinking about nothing' is the wordplay.
'one thinking' becomes 'muser' (someone who muses about things).
'about' indicates putting letters inside.
'nothing' becomes 'O' (O resembles 0).
'muser' placed around 'o' is 'MOUSER'.
(Other definitions for mouser that I've seen before include "One getting rid of vermin" , "Cat with certain skills" , "pest controller" , "Cat that's good at catching small rodents" , "Cat as hunter of small rodents" .)