Complex sentence: Letters 16 and 17 form a two-letter word ending in "P"; this has to be "UP"; hence, letter 16 is a "U", which can be filled into the appropriate clue answer in the list of clues.
Split into simpler sentences: Letters 16 and 17 form a two-letter word ending in "P". Since this has to be "UP", letter 16 is a "U", which can be filled into the appropriate clue answer in the list of clues.