One such word is whenever.
airtimeanytimebedtimedowntimelunchtimemealtimeovertimeplaytimesometimespringtimesummertimetimebasetimeouttimesharewintertime
roommate
a compound word beginning with cart is any word that begins with cart and has another word in it.
Some compound words starting with under include:underbrushunderclothesundercookundercoverundercutunderdoneunderestimateundergoundergroundundergrowthunderdevelopedunderhandedunderlineunderlyingundermineundernourishedunderpaidunderpantsunderpinningunderplayunderperformedunderratedundershortsundershotundersideunderstandunderstateunderstoodundertakeundertowunderusedunderwear
pop quiz
Fallback is a compound word. Falloff and fallout are compound words.
The answer is become.
A compound word starting with "drop" is "dropkick."
Doable
One such word is whenever.
zebralicious
airtimeanytimebedtimedowntimelunchtimemealtimeovertimeplaytimesometimespringtimesummertimetimebasetimeouttimesharewintertime
Balloon.
backbonebackgroundballparkbaseballbaseboardbasketballbathhousebathtubbathwaterbirdhousebirthdaybirthmarkbirthrightblackbirdblackboardblackmailbloodbathbookmarkbookshelfbluebellbluebirdblueprintboyhoodbrainwavebreadwinnerbreakwaterbrownstonebutterball
maidness
Bankteller