Old stone for German POW camp (5)
I believe the answer is:
oflag
'german pow camp' is the definition.
'old stone' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'stone' becomes 'flag' (flag is a kind of stone).
'o'+'flag'='OFLAG'
'for' acts as a link.
(Other definitions for oflag that I've seen before include "now-disused camp" , "Place for detention" , "Colditz, for example" , "German prisoner of war camp for officers" , "prison camp" .)