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 wordplay.
'soldiers' becomes 'or' (Other Rank).
'leading' says to put 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 "Mouth-like opening", "Mouth of a bodily cavity", "The mouth of a cavity", "Vent", "Hole in the head, perhaps?", "Aperture", "Hole, opening", "Finding opening", "Opening" and "Space".)