It's half-day closing mate! (6)
I believe the answer is:
friend
'mate' is the definition.
(I've seen this before)
'it's half-day closing' is the wordplay.
I cannot really understand how this works, but
'day' could be 'fri' (abbreviation for Friday) and 'fri' is found in the answer.
'closing' could be 'end' (I've seen this in another clue) and 'end' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for friend that I've seen before include "one quaking in society?" , "Colleague" , "Supporter of cause" , "Member of religious society" , "One who gives proof of genuine regard" .)