Old soldiers behind walls in Germany, I say! (3,4)
I believe the answer is:
red army
'old soldiers' is the definition.
(army of the USSR)
'behind walls in germany i say' is the wordplay.
'behind' becomes 'rear' (both can mean one's bottom).
'walls in' means one lot of letters goes inside another (I've seen this in other clues).
'germany' becomes 'd' (vehicle registration code for Deutschland).
'i say' becomes 'my' (both are exclamations of surprise).
'rear' placed around 'd' is 'redar'.
'redar'+'my'='RED ARMY'
(Other definitions for red army that I've seen before include "Revolutionary fighters" , "force" , "Cold War divisions?" , "Soviet troops" , "Left soldiers" .)