One might pass through customs in Germany chewing a Big Mac with pilot (10,3)
I believe the answer is:
diplomatic bag
'one might pass through customs' is the definition.
(customs officers cannot open a diplomatic bag)
'germany chewing a big mac with pilot' is the wordplay.
'germany' becomes 'd' (vehicle registration code for Deutschland).
'chewing' is an anagram indicator.
'with' says to put letters next to each other.
'a'+'big'+'mac'='abigmac'
'abigmac' after 'pilot' is 'pilotabigmac'.
'pilotabigmac' with letters rearranged gives 'iplomaticbag'.
'd'+'iplomaticbag'='DIPLOMATIC BAG'
'in' is the link.
(Other definitions for diplomatic bag that I've seen before include "Embassy's mail container" , "Embassy mail sack" , "One taking post abroad" , "Embassy mail pouch" .)