Possible Clues:

• Checks
• Discourages or prevents
• Puts off having rested like this
• To have five hundred trees chopped up is enough to put you off
• Forestalls
• Prevents
• This puts one off five hundred chopped-up trees
• How, perhaps, it's 21 across's turn to be the one that puts off what belongs to 'er
• Discourages or hinders
• The chopping up of five hundred trees is so off-putting