Obtains a different form of defence (7)
I believe the answer is:
bastion
'form of defence' is the definition.
(I know that bastion is a type of defence)
'obtains a different' is the wordplay.
'a different' is an anagram indicator.
'obtains' anagrammed gives 'BASTION'.
(Other definitions for bastion that I've seen before include "Fortified place near hostile territory" , "Obtains in fortified place or stronghold" , "Stronghold, fortification" , "part of stronghold" , "Projecting part of a fortification" .)