Without outside assistance, Head Office going to be engaged (2,3)
I believe the answer is:
in use
'be engaged' is the definition.
(I've seen this before)
'without outside assistance head office going' is the wordplay.
'without outside assistance' becomes 'inhouse' (performed internally by an organisation).
'head office' becomes 'ho'.
'going' is a deletion indicator.
'inhouse' with 'ho' taken out is 'IN USE'.
'to' is the link.
(Other definitions for in use that I've seen before include "Being employed - unavailable" , "taken?" , "Engaged" , "Current" , "Currently employed" .)