Mad piranhas as terrible as fleet of warships (7,6)
I believe the answer is:
spanish armada
'fleet of warships' is the definition.
(armada sent to England in 1588)
'mad piranhas as terrible' is the wordplay.
'terrible' is an anagram indicator.
'mad'+'piranhas'+'as'='madpiranhasas'
'madpiranhasas' with letters rearranged gives 'SPANISH ARMADA'.
'as' is the link.
(Other definitions for spanish armada that I've seen before include "Fleet sent against England in 1588" , "1588 invasion force" , "Famous naval fleet" , "Fleet that sailed from Lisbon in May fifteen eighty eight" , "1588 invasion fleet" .)