Details in command at Arnhem (4)
I believe the answer is:
data
'details' is the definition.
(I've seen this in another clue)
'in command at arnhem' is the wordplay.
'in' indicates the answer is hidden within the clue.
'DATA' is hidden within 'command at arnhem'.
(Other definitions for data that I've seen before include "Evidence" , "information from bank" , "Results of research" , "Input; facts" , "Facts and figures as processed by computer" .)