Moving east, the navy is behind (6)
I believe the answer is:
astern
'is behind' is the definition.
(I've seen this before)
'moving east the navy' is the wordplay.
'moving' indicates an anagram.
'the navy' becomes 'rn' (abbreviation for Royal Navy).
'east' with letters rearranged gives 'aste'.
'aste'+'rn'='ASTERN'
(Other definitions for astern that I've seen before include "at the vessel's rear" , "State of S central US" , "Ran set (anag.)" , "back for sailors" , "Backward" .)