Fallback is a compound word. Falloff and fallout are compound words.
The answer is become.
A compound word starting with "drop" is "dropkick."
airtimeanytimebedtimedowntimelunchtimemealtimeovertimeplaytimesometimespringtimesummertimetimebasetimeouttimesharewintertime
One such word is whenever.
zebralicious
Balloon.
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maidness
Bankteller
mousehouse
Snail