Up-to-date fashion with the navy (6)
I believe the answer is:
modern
'up-to-date' is the definition.
(synonyms)
'fashion with the navy' is the wordplay.
'fashion' becomes 'mode' (synonyms).
'with' is a charade indicator (letters next to each other).
'the navy' becomes 'RN' (abbreviation for Royal Navy).
'mode'+'rn'='MODERN'
(Other definitions for modern that I've seen before include "See 11" , "Recent, up to date" , "In history, the period or era since the Middle Ages" , "Of the present time or current fashion" , "Belonging to the present time" .)