aster, astor, blaster, caster, castor, disaster, faster, grandmaster, grandmaster, master, mcmaster, oleaster, paster, pastor, pilaster, plaster, raster.
Your query has returned 724 words, which include anagrams of headmasters as well as other shorter words that can be made using the letters included in headmasters.
The words in this list can be used in games such as Scrabble, Words with Friends, Wordscapes and other similar games.