A friend means everything, at the end of the day (4)
I believe the answer is:
ally
'a friend' is the definition.
(an ally is a friend or supporter)
'everything at the end of the day' is the wordplay.
'everything' becomes 'all' (all things).
'at the end of' indicates one should take the final letters.
The last letter of 'day' is 'y'.
'all'+'y'='ALLY'
'means' is the 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" .)