In favour of turning up at mid-afternoon (3)
I believe the answer is:
for
'in favour' is the definition.
(as in being for a particular cause)
'of turning up at mid-afternoon' is the wordplay.
'turning up' says the letters should be written backwards.
'at' says to put letters next to each other.
'mid' indicates the central letters.
The middle letter of 'afternoon' is 'r'.
'of' backwards is 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "In favour of; since" , "On behalf of" , "Approving" , "Favouring" , "On the side of" .)