New fashion navy (6)
I believe the answer is:
modern
'new' is the definition.
(associated in meaning)
'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 "The very latest available" , "Present-day" , "Recent, up to date" , "Novel" , "Fashionable (like the new Tate gallery?)" .)