In a plot, a bit of thinking that's overdue (6)
I believe the answer is:
behind
'overdue' is the definition.
(thesaurus)
'in a plot a bit of thinking' is the wordplay.
'in' means one lot of letters goes inside another.
'a plot' becomes 'bed' (a plot of land).
'a bit of thinking' becomes 'hin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bed' placed around 'hin' is 'BEHIND'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for behind that I've seen before include "Following - late" , "Taking place in secret" , "Hidden by; bottom" , "in van? No" , "Running late" .)