A Royal marine promotion, with a lot of ships (6)
I believe the answer is:
armada
'lot of ships' is the definition.
'a royal marine promotion with a' is the wordplay.
'royal marine' becomes 'RM' (abbreviation).
'promotion' becomes 'ad' (short for advert).
'with' says to put letters next to each other.
'a'+'rm'+'ad'+'a'='ARMADA'
(Other definitions for armada that I've seen before include "A great fleet - Spain had one" , "Fighting unit" , "Naval fighting force" , "force of the sea" , "Navy" .)