A friend means everything, at the end of the day (4)
I believe the answer is:
ally
'a friend' is the definition.
('ally' can be a synonym of 'friend')
'everything at the end of the day' is the wordplay.
'everything' becomes 'all' (I've seen this before).
'at the end of' says to take the final letters.
The last letter of 'day' is 'y'.
'all'+'y'='ALLY'
'means' acts as a link.
(Other definitions for ally that I've seen before include "Co-operating person or nation" , "Friend in time of need" , "support" , "A friendly and reliable country" , "He fights on your side" .)