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' means one lot of letters go next to another.
'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" .)