Vehicle reversing by a line of boats (5)
I believe the answer is:
naval
'of boats' is the definition.
(referring to a navy)
'vehicle reversing by a line' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle).
'reversing' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'line' becomes 'l'.
'van' backwards is 'nav'.
'nav'+'a'+'l'='NAVAL'
(Other definitions for naval that I've seen before include "Relating to the Senior Service" , "Of seaborne forces" , "Maritime" , "Relating to nautical defence forces" , "Relates to nautical defence force" .)