Exist alongside tree, perhaps
I believe the answer is:
maybe
'perhaps' is the definition.
('maybe' can be a synonym of 'perhaps')
'exist alongside tree' is the wordplay.
'exist' becomes 'be' ('be' can be a synonym of 'exist').
'alongside' says to put letters next to each other.
'tree' becomes 'may' (I am not sure about this - if you are sure you should believe this answer much more).
'be' put after 'may' is 'MAYBE'.
Can you help me to learn more?
(Other definitions for maybe that I've seen before include "Perhaps, might happen" , "Perhaps, possibly" .)