Discover or expel crook (7)
I believe the answer is:
explore
'discover' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their base form.
Perhaps there's an association between them I don't understand?
'or expel crook' is the wordplay.
'crook' is an anagram indicator (a crooked or broken form of the letters).
'or'+'expel'='orexpel'
'orexpel' with letters rearranged gives 'EXPLORE'.
Can you help me to learn more?
(Other definitions for explore that I've seen before include "Examine unknown territory" , "Look around; examine" , "Travel through region to learn about it" , "Search and inquire into" , "Travel in search of discovery" .)