In addition to the information given by andys, there are several conventions used by crossword-setters that would be very hard to guess. For example, they sometimes use the word 'swan' to indicate the letters PEN (because a female swan is a pen, apparently); then there's 'sailor' which could indicate the letters AB (for able-bodied [seaman]) or TAR (for [jolly-jack] tar). I expect there are several others.
And lots of one letter abbreviations: g using good, f - fine, n - name, b - born, d - died, etc, or Roman numerals - M - 1,000, X - ten, etc. Also, where words are hidden within the clue, e.g. "cheese made in reverse" = edam ("made" backwards). My last clue, used two synonyms and a homonym and a play on sentence structure - that's my clue....