At the end of the day, it's all over, friend (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'at the end of the day it's all' is the wordplay.
'at' says to put letters next to each other.
'the end of the' says to take the final letters.
The final letter of 'day' is 'y'.
'y' put after 'all' is 'ALLY'.
'over' acts as a link.
(Other definitions for ally that I've seen before include "Team up" , "Confederate; supporter" , "support" , "One on one's side" , "Supporter in a conflict" .)