First man from south to join a king's fleet (6)
I believe the answer is:
armada
'fleet' is the definition.
(armada is a kind of fleet)
'first man from south to join a king's' is the wordplay.
'first man' becomes 'adam' (I've seen this before).
'from south' says the letters should be written backwards.
'to join' means one lot of letters go next to another.
'king' becomes 'r' (r is abbreviation for rex).
'adam' backwards is 'mada'.
'a'+'r'='ar'
'mada' put after 'ar' is 'ARMADA'.
(Other definitions for armada that I've seen before include "Spanish war fleet" , "A great fleet - Spain had one" , "Large number of ships (Spanish?)" , "sea-borne unit" , "Large war fleet" .)