21D does seem pretty unfair. "[Chambers] does not give the proper name at 21." OK, but no other reference work I've got does either. The clue is Classical tyrant, Greek character imprisoned by demigod. There is a gimmick: the correct answer (5 letters) has an extra P. The wordplay gives the 6-letter form with the P in it. Clearly the answer is PI inside a demigod, and the whole thing is a classical tyrant. Checking letters give -PIERO. I suppose it could be construed as PI inside classical tyrant (NERO) giving demigod as the definition. NIERO? As I say, pretty unfair.