Defender in boats at sea (7)
I believe the answer is:
bastion
'defender' is the definition.
(I've seen this before)
'in boats at sea' is the wordplay.
'at sea' indicates an anagram.
'in'+'boats'='inboats'
'inboats' with letters rearranged gives 'BASTION'.
(Other definitions for bastion that I've seen before include "staunch supporter" , "that's fortified" , "Obtains (anag.)" , "Stronghold, fortification" , "Obtains in fortified place or stronghold" .)