Might be unfriendly, going out of turn (5)
I believe the answer is:
could
'might 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 you can see an association between them that I can't see?
'unfriendly going out of turn' is the wordplay.
'unfriendly going' becomes 'cold' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'out of' indicates putting letters inside.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'cold' placed around 'u' is 'COULD'.
Can you help me to learn more?
(Another definition for could that I've seen is " Was able to (do something)".)