Progressive kind joining Royal Navy (6)
I believe the answer is:
modern
'progressive' is the definition.
('modern' can be similar in meaning to 'progressive')
'kind joining royal navy' is the wordplay.
'kind' becomes 'mode'.
'joining' is a charade indicator (letters next to each other).
'royal navy' becomes 'rn'.
'mode'+'rn'='MODERN'
(Other definitions for modern that I've seen before include "Activity of 18 ac" , "Up to date, of present times" , "In history, the period or era since the Middle Ages" , "The very latest available" , "See 11" .)