In favour of returning to the air terminal (3)
I believe the answer is:
for
'in favour' is the definition.
(I've seen this before)
'of returning to the air terminal' is the wordplay.
'returning' is a reversal indicator.
'to' means one lot of letters go next to another.
'terminal' says to take the final letters (terminal refers to the end).
The last letter of 'air' is 'r'.
'of' back-to-front is 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "On behalf of" , "In favour of; since" , "and 16 and 20: Notwithstanding" , "Favouring" , "Pro" .)