In favour of returning at mid-April (3)
I believe the answer is:
for
'in favour' is the definition.
(as in being for a particular cause)
'of returning at mid-april' is the wordplay.
'returning' shows that the letters should be reversed in order.
'at' means one lot of letters go next to another.
'mid' means to look at the middle letters (mid- can mean the middle of something).
The central letter of 'april' is 'r'.
'of' reversed gives 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "Approving" , "Pro" , "See 24" , "Favouring" , "and 16 and 20: Notwithstanding" .)