Hi, Adeano,
No need to apologise. I see that several regulars have identified this as a very difficult puzzle. Looking back over 2022, the only puzzle I gave up on was No. 4717 ("Unruly Characters" by Somniloquist on 25 June 2022), but I managed to finish all of the others by Saturday, and mainly by Friday evening, except for this one. I didn't finish it until Sunday lunchtime, and only by realising that the second sentence of the instruction - "In some cells letters from two or more entries clash . . ." - did not restrict the number of letters from any one of the clashing entries to just a single letter. Once I saw that, trying to fit the entry for 3W into the grid, the penny dropped, and many "clashes" that I'd pencilled in elsewhere turned out not to be clashes after all.
Over the last two or three years, there have been several puzzles involving multiple clashing letters in a single cell, which need to be rearranged into a word that is often replaced by a symbol. For example No. 4644 ("Symbols" by Aedites, on 30 January 2021) involved the sets of clashing letters from the across and down entries leading to the names of the twelve signs of the zodiac , which had to be replaced by the relevant symbols. To be fair, though, in the preamble for that puzzle it's fairly clear that there will be several letters from at least one of the clashing entries in the clashing cell.
So I don't think that the key instruction is missing, it's just (I suspect deliberately) vague.