Defender obtains ground (7)
I believe the answer is:
bastion
'defender' is the definition.
(I've seen this before)
'obtains ground' is the wordplay.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'obtains' with letters rearranged gives 'BASTION'.
(Other definitions for bastion that I've seen before include "Obtains (anag.)" , "staunch supporter" , "Stronghold, fortification" , "Projecting part of a fortification" , "that's fortified" .)