U-boat could be somewhere nearby (5)
I believe the answer is:
about
'somewhere nearby' is the definition.
(about can mean nearby or around)
'u-boat could be' is the wordplay.
'could be' indicates an anagram.
'u'+'boat'='uboat'
'uboat' is an anagram of 'ABOUT'.
(Other definitions for about that I've seen before include "Approximately, around" , "Concerning; approximately" , "Concerning or around" , "Concerning (something)" , "Concerning or in neighbourhood" .)