Architect, revolutionary, built new enclosure for king and (previously) emperor initially (8)

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

A sort-of envelope (‘enclosure for’) of E (’emperor’) and K (‘king’) separately, and with ‘previously’ indicating the order, in LUBTI, an anagram (‘revolutionary’) of ‘built’; plus N (‘new’). Berthold Lubetkin was the 1930s modernist designer of the Round House and Penguin Pool in London Zoo. As King and Emperor are species of penguin, the clue has a brilliant extended definition.