One in favour of half-day closing (6)
I believe the answer is:
friend
'one in favour of half-day closing' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'day closing' is the wordplay.
'day' becomes 'fri' (abbreviation for Friday).
'closing' becomes 'end' (I've seen this in another clue).
'fri'+'end'='FRIEND'
Can you help me to learn more?
(Other definitions for friend that I've seen before include "Organisation's supporter" , "Pal, mate" , "one quaking in society?" , "Close associate" , "Buddy, chum" .)