This uses charade (joining word parts) combined with deletion - look for parts that can be joined together, with something being removed or 'lost'
Find who the 'setter' refers to in cryptic crossword terms, then look at 'was one lost' - this means take the letters of 'was' but remove 'one' (the letter 'a'), then combine the parts
Answer: MEWS | ME (the setter/compiler of the crossword) + WS (from 'was' with 'one'=A removed/lost) = MEWS