Mate who's close by day (6)
I believe the answer is:
friend
'mate' is the definition.
(mate can mean a friend)
'close by day' is the wordplay.
'close' becomes 'end' (close can mean to end or finish).
'by' means one lot of letters go next to another.
'day' becomes 'fri' (abbreviation for Friday).
'end' put after 'fri' is 'FRIEND'.
'who's' is the link.
(Other definitions for friend that I've seen before include "allowed to pass by the guard" , "Close associate" , "Member of a religious community" , "One you know well and regard with affection" , "Quaker - pal" .)