First man, artist back in naval force (6)
I believe the answer is:
armada
'naval force' is the definition.
(I've seen this before)
'first man artist back' is the wordplay.
'first man' becomes 'adam' (first man in the Bible).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'back' shows that the letters should be reversed in order.
'adam'+'ra'='adamra'
'adamra' backwards is 'ARMADA'.
'in' acts as a link.
(Other definitions for armada that I've seen before include "sea-borne unit" , "Large Spanish fleet" , "A great fleet, ships or planes" , "Navy" , "Large number of ships (Spanish?)" .)