Arsenic? Sure to suss out (9)
I believe the answer is:
ascertain
'suss out' is the definition.
The answer and definition 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?
'arsenic? sure' is the wordplay.
'arsenic?' becomes 'As' (As is the chemical symbol for arsenic).
'sure' becomes 'certain' (synonyms).
'as'+'certain'='ASCERTAIN'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for ascertain that I've seen before include "Learn" , "settle" , "Find out by making enquiries in sectarian way" , "Get to know" , "Prove" .)