Concerning the U-boat wreck (5)
I believe the answer is:
about
'concerning' is the definition.
(about can mean concerning or relating to)
'u-boat wreck' is the wordplay.
'wreck' indicates anagramming the letters.
'u'+'boat'='uboat'
'uboat' is an anagram of 'ABOUT'.
'the' acts as a link.
(Other definitions for about that I've seen before include "Circa" , "in action" , "Concerning or around" , "Concerning (something)" , "In the area of or in relation to" .)