Fish
bust base busk busy bush
seesaw
gusset
Sites
sepsis
pets
ladybirds, calyptras, kalyptras, capybaras
pens lens legs
Stag
There are over 100 words that can fit this:AramesBracesBrachsBractsBraidsBrailsBrainsBrakesBrandsBranksBrantsBravasBravesBravosBrawlsBrawnsBrazasBrazesCraalsCracksCraftsCrakesCrampsCranesCranksCrapesCrasesCrasisCratesCravesCrawlsCrazesDraffsDraftsDrailsDrainsDrakesDramasDrapesDrawlsErasesFracasFrailsFramesFrancsFranksFraudsGraalsGracesGradesGradusGraftsGrailsGrainsGramasGrampsGrandsGrantsGrapesGraphsGraspsGratesGratisGravesGrazesIradesKraalsKraftsKraitsKrautsOrangsOratesPraamsPrahusPrangsPranksPrasesPratesPrawnsPraxesPraxisTracesTracksTractsTradesTragusTraiksTrailsTrainsTraitsTrampsTranksTranqsTrapesTravesTrawlsUraeusUraresUrarisUrasesUratesWracksWrangsWraths
The word could be execs.
Some choices for _as_er are:bastercasterdashereasierEasterfasterjaspermashermasterrashertastervasterwasherwaster