Possible Clues:

• Place too great a burden on, as Revenue officials do
• Strain too much to have them above little nails, by the sound of them (7)
• Impose too much strain on
• Try too hard to get above the nails, by the sound of it
• Weary
• Require too much
• Place too great a burden on nails, say
• Put too great a strain on, as Revenue Commissioners do
• Fleece to wear out
• Try to do too much at the cleric up in there when there's an ox around