Ex-boozers acquiring unknown pub to host market (6)

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

An envelope (‘to host’) of Z (mathematical ‘unknown’) plus (‘acquiring’) AA (Alcoholics Anonymous, ‘ex-boozers’) in BAR (‘pub’). The word ordering of ‘acquiring’ and ‘to host’ is tortuous, but not impossible. We have to go back to Wednesday’s Carpathian for the same answer (and definition), with different wordplay.