Mounted soldiers covering old actor's way in (5,4)
I believe the answer is:
stage door
'actor's way in' is the definition.
'mounted soldiers covering old' is the wordplay.
'mounted' becomes 'staged'.
'soldiers' becomes 'or' (Other Rank).
'covering' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'or' going around 'o' is 'oor'.
'staged'+'oor'='STAGE-DOOR'
(Other definitions for stage door that I've seen before include "Film" , "Artist's entrance" , "sign autographs here?" , "What players have to go through" , "Theatre back entrance" .)