Bring up sappers and gunners facing backwards (4)
I believe the answer is:
rear
'bring up' is the definition.
(rearing is a kind of bringing up)
'sappers and gunners facing backwards' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'and' means one lot of letters go next to another.
'gunners' becomes 'ra' (Royal Artillery).
'facing backwards' says the letters should be written backwards.
'ra' written backwards gives 'ar'.
're'+'ar'='REAR'
(Other definitions for rear that I've seen before include "Buttocks - rise up" , "Posterior" , "Rise up at the back" , "what can be brought up, finally" , "Stern" .)