A drama about some vessels (6)
I believe the answer is:
armada
'some vessels' is the definition.
(I've seen this before)
'a drama about' is the wordplay.
'about' indicates an anagram.
'drama' anagrammed gives 'rmada'.
'a'+'rmada'='ARMADA'
(Other definitions for armada that I've seen before include "Large Spanish fleet" , "1588 sea battle" , "Large war fleet" , "Old Spanish fleet" , "force of the sea" .)