Any compound word counts as one word
Allow compound words in every written form as one-word clues: pack rat and mother-in-law are legal, not just greenhouse. The test is the dictionary — if it is listed as a headword in any of those forms, it counts. That keeps invented compounds out, since a made-up pairing like lunar squid for MOON and OCTOPUS is nobody's headword, and it settles the harder question the loosening creates: under the printed default you only ever ask whether a clue is written as one word, but once two-word compounds are legal somebody will ask whether swimming pool or board game is a compound at all. The dictionary answers that at the table instead of the table arguing it.
