Three such words are laptop, lapboard and lapdog.
Some compound words starting with under include:underbrushunderclothesundercookundercoverundercutunderdoneunderestimateundergoundergroundundergrowthunderdevelopedunderhandedunderlineunderlyingundermineundernourishedunderpaidunderpantsunderpinningunderplayunderperformedunderratedundershortsundershotundersideunderstandunderstateunderstoodundertakeundertowunderusedunderwear
houseboathouseboyhousebreakhousemaidhouseholdhousebrokenhousecoathousewareshouseworkhousecleaninghousewifehouseplanthousematehousetophousewarming
Fallback is a compound word. Falloff and fallout are compound words.
The answer is become.
teardrop gumdrop raindrop dewdrop backdrop
airtimeanytimebedtimedowntimelunchtimemealtimeovertimeplaytimesometimespringtimesummertimetimebasetimeouttimesharewintertime
zebralicious
One such word is whenever.
Doable
compound words some- * sometime * somebody * something * someday * somehow * someone * someplace * somersault * somewhat * somewhere
Balloon.
mousehouse