U-boat on the move nearby (5)
I believe the answer is:
about
'nearby' is the definition.
(synonyms)
'u-boat on the move' is the wordplay.
'on the move' indicates an anagram.
'u'+'boat'='uboat'
'uboat' is an anagram of 'ABOUT'.
(Other definitions for about that I've seen before include "in the offing?" , "Concerning (something)" , "'All - - Eve' (film)" , "Concerning or in neighbourhood" , "Approximately, around" .)