Defender messing about in boats (7)
I believe the answer is:
bastion
'defender' is the definition.
(I've seen this before)
'messing about in boats' is the wordplay.
'messing about' indicates an anagram.
'in'+'boats'='inboats'
'inboats' is an anagram of 'BASTION'.
(Other definitions for bastion that I've seen before include "Defence - in boats (anag)" , "Obtains (anag.)" , "Fortified place near hostile territory" , "Obtains in fortified place or stronghold" , "'Stronghold, citadel (7)'" .)