Possible Clues:

• Two aliens, one with no answer for hard taskmaster
• Swallow, visitor from outer space, a disciplinarian!
• Disciplinarian in term at school
• A disciplinarian, I rant terribly in a police force
• 'Disciplinarian, exacts strict conformity to rules (8)'
• Strict taskmaker
• Rigorous figure having some point in the market
• Met train (anag.)
• A disciplinarian involved in matter
• He's strict, neat and trim, surprisingly