Surmise column, one penned (5)
I believe the answer is:
posit
'surmise' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their base form.
Maybe you can see an association between them that I don't see?
'column one penned' is the wordplay.
'column' becomes 'post' (I have seen 'A column' mean 'post' so perhaps 'column' could also mean 'post').
'one' becomes 'I' (Roman numeral).
'penned' is an insertion indicator (inserted letters are enclosed or penned in).
'post' placed around 'i' is 'POSIT'.
Can you help me to learn more?
(Other definitions for posit that I've seen before include "advance" , "Propose" , "Suggest" , "Put forward as basis for discussion" , "Put forward; place" .)