The clues in Guardian crosswords seem to be made as far from the standard usage as possible without being impossible to say it's the same word. Which makes sense given their function. Though I do hope nobody's trying to learn English from them.
Most crossword compilers use Chambers as their dictionary of choice - mainly because of its liberal attitude towards the inclusion of variant spellings and secondary meanings. I also have on my bookshelf a copy of the Chambers Crossword Dictionary, which includes plentiful lists of common cruciverbalisms.
The only thing you will learn from doing crosswords regularly is how to become good at solving crosswords.
Brewers Dictionary of Phrase & Fable is also a classic crossword companion.