A dog, perhaps, under standard defensive structure (7)
I believe the answer is:
parapet
'defensive structure' is the definition.
(parapet is a kind of defensive structure)
'a dog perhaps under standard' is the wordplay.
'dog perhaps' becomes 'pet'.
'under' is a charade indicator (letters next to each other).
'standard' becomes 'par' (standard of play in golf).
'a'+'pet'='apet'
'apet' put after 'par' is 'PARAPET'.
(Other definitions for parapet that I've seen before include "Defensive line" , "balustrade" , "form of protection" , "place of safety?" , "wall, protective one" .)