Authorisation in advance (4)
I believe the answer is:
pass
This clue is a double definition.
'authorisation' is the first definition.
(eg a security pass)
'advance' is the second definition.
(passing is a kind of advancing)
'in' acts as a link.
(Other definitions for pass that I've seen before include "change" , "complimentary ticket" , "Spend (19, say) -- don't know" , "Pronounce (judgement)" , "What a hooker may do" .)