U-boat damaged, initially holed in a north German route (8)
I believe the answer is:
autobahn
'german route' is the definition.
(German term for a motorway)
'u-boat damaged initially holed in a north' is the wordplay.
'damaged' indicates an anagram.
'initially' suggests taking the first letters.
'in' is an insertion indicator.
'north' becomes 'n' (synonyms).
The first letter of 'holed' is 'h'.
'u'+'boat'='uboat'
'uboat' is an anagram of 'utoba'.
'utoba'+'h'='utobah'
'a'+'n'='an'
'utobah' placed inside 'an' is 'AUTOBAHN'.
(Other definitions for autobahn that I've seen before include "expressway" , "Motorway in Germany" , "Great way" , "way abroad" , "One way the Germans drive" .)