Be proper to ape in trap (9)
I believe the answer is:
appertain
'be' is the definition.
The answer and definition can be both to do with fundamental existence as well as being verbs in their base form.
Maybe there's an association between them I don't understand?
'proper to ape in trap' is the wordplay.
'ape in trap' anagrams to 'APPERTAIN'.
Though, I'm unsure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for appertain that I've seen before include "Bear on" , "Concern" , "Belong, relate (to)" , "Be relevant to" .)