A drama about ships (6)
I believe the answer is:
'ships' is the definition.
(group of ships)
'a drama about' is the wordplay.
'about' indicates an anagram.
'drama' is an anagram of 'rmada'.
(Other definitions for armada that I've seen before include "Naval fighting force" , "1588 sea battle" , "Fleet of armed ships" , "vessels" , "force of the sea" .)