Soldiers, if leading reserve, will get opening (7)
I believe the answer is:
Here is my best explanation:
'opening' is the definition.
('orifice' can be a synonym of 'opening')
'soldiers if leading reserve' is the subsidiary indication.
'soldiers' becomes 'or' (Other Rank).
'leading' is a charade indicator (letters next to each other).
'reserve' becomes 'ice' (I've seen this before).
'will get' is the link.
(Other definitions for orifice that I've seen before include "Hole, opening", "The mouth of a cavity", "A bodily opening", "Mouth-like opening", "For CIE, I look for an opening", "Hole in the head, perhaps?", "Bodily opening, mouth of cavity", "Opening", "A bodily cavity" and "Mouth of a bodily cavity".)