Definitions in relation to their use in these kangaroo words, taken from Google Dictionary, and edited for formatting.
Kangaroo word: feast
Joey word: fast
feast (verb): eat and drink sumptuously.
fast (verb): abstain from all or some kinds of food or drink, especially as a religious observance.
The letters F, A, S, and T found within the anti-kangaroo word “feast” arrange in the listed order to complete the joey word “fast”. While a word can have multiple definitions, in relation to their use with kangaroo words, these two words share an opposite meaning: eat food vs. eat nothing.