Pin down, as in trace perhaps (9)
I believe the answer is:
ascertain
'pin down' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?
'as in trace perhaps' is the wordplay.
'perhaps' indicates an anagram.
'in'+'trace'='intrace'
'intrace' anagrammed gives 'certain'.
'as'+'certain'='ASCERTAIN'
Can you help me to learn more?
(Other definitions for ascertain that I've seen before include "Find out for sure, determine" , "Find out by making enquiries in sectarian way" , "Confirm, make sure" , "Prove" , "Get to know" .)