To use an expression to control label rotation, choose "edit" from the data defined override menu.

If all of your angles are positive numbers, use this expression to make them negative:

concat('-', 'fieldname') 

If your angles include positive and negative values, use this expression:

'fieldname' * -1

To use an expression to control label rotation, choose "edit" from the data defined override menu.

If all of your angles are positive numbers, use this expression to make them negative:

concat('-',  to_real('fieldname')) 

If your angles include positive and negative values, use this expression:

 to_real('fieldname') * -1

If your field is already in numerical format you can leave out the to_real() function.