avast, boast, coast, deist, exist, foist, ghost, hoist, joist
accustomedalabasterbreastbonebastedcastlecustomcostumeclusterdistresseasternflusterfastergamgsterhamsterhostageimpostorjusticelusterlistenmastermistermustlemusternestlepistolpastequestionrustlesemstersistertrustworthyvistagewasteyesterdayzesty
Some four letter words that start with ST are star, step, stop, and stay.
firstlastoftestblacklistcroakiest
star
stabstaffstagstaggeredstagestalkstallstalestimulatestampedestainstandstarstockstonestorestolestipulatestrangledstrugglestrandstepstopstuckstorkstickstackstemsteamstaplestarkstatestoopstovestumblestiffstuffstethoscopestereostylestrangestringstrikesturdystudysteadystampstompsteepstealsteelstewstowsteakstakestrapstripestrobestrivestarvestooge
start
The only one I could find was Utas - the eighth day after any term or feast; as, the utas of St. Michael.
There are several 5 letter words that start with st. Some of them are; start, stage and staff.
stastist
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern ST--END. That is, seven letter words with 1st letter S and 2nd letter T and 5th letter E and 6th letter N and 7th letter D. In alphabetical order, they are: stipend
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern A--ST--ST. That is, nine letter words with 1st letter A and 4th letter S and 5th letter T and 8th letter S and 9th letter T. In alphabetical order, they are: angstiest
Superlatives end in -st, -est or contain the words most or least. Look for the one that doesn't do that.