Officer on street blocking large expenditure items (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

[ NCO(abbrev. for “non-commissioned officer”) placed above ( on , in a down clue) ST(abbrev. for “street”) ] contained in ( blocking ) OS(abbrev. for “outsize”/large). Defn: …, in this case, overhead costs.