Antique unit to get angry about state museum's leader (5,4)

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

OMAN (state) Museum in RILE (to get, i.e., make angry); a Roman mile (mille passuum or milia passuum – a thousand paces – from which ‘mile’ is derived) was somewhat shorter than a modern mile, standardised in 29BC by Agrippa at around 1620 modern yards. Well, you know, nutrition being what it was, those legionaries would have had shorter legs.