Back, sanction (7)
I believe the answer is:
endorse
This clue is a double definition.
'back' is the first definition.
(back can mean to support or endorse)
'sanction' is the second definition.
(endorsing is a kind of sanctioning)
(Other definitions for endorse that I've seen before include "Approve officially" , "Give one's approval to" , "Red ones (anag.)" , "No reeds (anag)" , "Approve of, sign on back as of cheque" .)