Trendy fashion, navy (6)
I believe the answer is:
modern
'trendy' is the definition.
(thesaurus)
'fashion navy' is the wordplay.
'fashion' becomes 'mode' (synonyms).
'navy' becomes 'RN' (abbreviation for Royal Navy).
'mode'+'rn'='MODERN'
(Other definitions for modern that I've seen before include "Belonging to the present or recent times" , "Activity of 18 ac" , "Novel" , "Of the present time or current fashion" , "See 11" .)