Thinker is probing the bit left over (8)
I believe the answer is:
theorist
'thinker' is the definition.
(I've seen this before)
'is probing the bit left over' is the wordplay.
'probing' indicates putting letters inside (probe can mean to poke or prod into).
'bit' becomes 'o' (resembles 0, a binary digit or bit).
'left over' becomes 'rt' (I can't justify this - if you can you should believe this answer much more).
'the'+'o'+'rt'='theort'
'is' put into 'theort' is 'THEORIST'.
Can you help me to learn more?
(Other definitions for theorist that I've seen before include "Expert in established hypotheses" , "An expert in abstract principles" , "Creator of hypothesis" , "Abstract reasoner" , "Speculator" .)