A bulwark in boats, perhaps (7)
I believe the answer is:
bastion
'a bulwark' is the definition.
(I've seen this in another clue)
'in boats perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'in'+'boats'='inboats'
'inboats' is an anagram of 'BASTION'.
(Other definitions for bastion that I've seen before include "Obtains (anag.)" , "Stronghold, fortification" , "Defence - in boats (anag)" , "part of stronghold" , "'Stronghold, citadel (7)'" .)