Defence in boats at sea (7)
I believe the answer is:
bastion
'defence' is the definition.
(I know that bastion is a type of defence)
'in boats at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'in'+'boats'='inboats'
'inboats' anagrammed gives 'BASTION'.
(Other definitions for bastion that I've seen before include "fortress" , "Stronghold, fortification" , "Defence - in boats (anag)" , "A projecting fortification to increase angle of fire" , "staunch supporter" .)