Backing soldiers, play guitar behind stage (7)
I believe the answer is:
rostrum
'stage' is the definition.
(I've seen this before)
'backing soldiers play guitar behind' is the wordplay.
'backing' says the letters should be written backwards.
'soldiers' becomes 'or' (Other Rank).
'play guitar' becomes 'strum' (I've seen this before).
'behind' means one lot of letters go next to another.
'or' backwards is 'ro'.
'ro'+'strum'='ROSTRUM'
(Other definitions for rostrum that I've seen before include "place for speaker" , "At which one may conduct" , "Platform for orchestra conductor" , "Platform for speaker" , "Podium" .)