Authorise advance (4)
I believe the answer is:
pass
I believe this is a double definition.
'authorise' is the first definition.
('pass' can be a synonym of 'authorise')
'advance' is the second definition.
(passing is a kind of advancing)
(Other definitions for pass that I've seen before include "Ratify for saps" , "I've no idea" , "Spend (19, say) -- don't know" , "What a hooker may do" , "Progress" .)