Stalag ground is filled with hostility (5)
I believe the answer is:
aggro
'hostility' is the definition.
(I've seen this before)
'stalag ground is filled with' is the wordplay.
'with' says the answer is hidden in the clue.
'AGGRO' can be found hidden inside 'stalag ground filled'.
(Other definitions for aggro that I've seen before include "Aggressive, violent behaviour in short" , "Difficulties (slang)" , "Difficulties (sl.)" , "offensive behaviour" , "In slang, hassle or troublemaking" .)