It's generally believed that the synonym (called a joey) should be the same part of speech as the kangaroo word and its letters should appear in order.
The term kangaroo word was popularized by author Ben L. O'Dell in a short article in The American Magazine, 1956.
Examples and Observations:
- "Why do we call them kangaroo words? Not because they originated in Australia. Rather, these are marsupial words that carry smaller versions of themselves within their spellings. So 'respite' has 'rest,' 'splotch' has 'spot,' 'instructor' has 'tutor,' and 'curtail' has 'cut.' Sometimes a kangaroo word has more than one joey. The word 'feasted' has a triplet, 'fed,' 'eat,' and 'ate.' Finally, two qualifications: the joey word has to have its letters in order within the parent kangaroo word, but if all the letters are adjacent, for example, enjoy/joy, it doesn't qualify."
(Anu Garg, Another Word a Day. Wiley, 2005)
- destruction (ruin)
- "Among the kangaroo words that yield the most joviality and joy are those that conceal multiple joeys. Let's now perambulate, ramble, and amble through an exhibit of this species. Open up a container and you get a can and a tin. When you have feasted, you ate and have fed. When you deteriorate, you rot and die. A routine is both rote and a rut. Brooding inside loneliness are both loss and oneness.
"A chariot is a car and a cart. A charitable foundation is both a fund and a font. Within the boundaries of a municipality reside city and unity, while a community includes county and city."
(Richard Lederer, The Word Circus: A Letter-Perfect Book. Merriam-Webster, 1997)