Vulnerable at the outset before crossing German border (5)
I believe the answer is:
verge
'border' is the definition.
(verge is a kind of border)
'vulnerable at the outset before crossing german' is the wordplay.
'at the outset' says to take the initial letters.
'before' becomes 'ere' (usually literary**).
'crossing' means one lot of letters goes inside another.
'german' becomes 'g' (abbreviation).
The first letter of 'vulnerable' is 'v'.
'ere' going around 'g' is 'erge'.
'v'+'erge'='VERGE'
(Other definitions for verge that I've seen before include "Grass border along road" , "Side of road" , "Grass edge to road" , "Margin or brink" , "Grass edging" .)