Hell merges with authoritarian (8)

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

DIS (hell = the underworld: from a Roman god of the underworld, and it’s also the name of a region of Hell in Dante’s Divine Comedy ) + STRICT (authoritarian), “merging” so that the two letters S overlap in the middle. Reference to 8a LINE: the District line of the London Underground.