Defender in boats destroyed (7)
I believe the answer is:
bastion
'defender' is the definition.
(I've seen this before)
'in boats destroyed' is the wordplay.
'destroyed' indicates an anagram.
'in'+'boats'='inboats'
'inboats' is an anagram of 'BASTION'.
(Other definitions for bastion that I've seen before include "Strong defender" , "staunch supporter" , "Defence - in boats (anag)" , "fortress" , "that's fortified" .)