Defence of messing about in boats (7)
I believe the answer is:
bastion
'defence' is the definition.
(bastion is a kind of defence)
'messing about in boats' is the wordplay.
'messing about' is an anagram indicator.
'in'+'boats'='inboats'
'inboats' with letters rearranged gives 'BASTION'.
'of' is the link.
(Other definitions for bastion that I've seen before include "Stronghold, fortification" , "A projecting fortification to increase angle of fire" , "Fortified place near hostile territory" , "Obtains in fortified place or stronghold" , "'Stronghold, citadel (7)'" .)