Associate day with purpose (6)
I believe the answer is:
friend
'associate' is the definition.
(friend is a kind of associate)
'day with purpose' is the wordplay.
'day' becomes 'fri' (abbreviation for Friday).
'with' means one lot of letters go next to another.
'purpose' becomes 'end' (both can mean a goal).
'fri'+'end'='FRIEND'
(Other definitions for friend that I've seen before include "Comrade for finder" , "allowed to pass by the guard" , "Pal, mate" , "Buddy, mate" , "China" .)