Only child, should have had another one or two to get more practice in!
Dad of 3 here, youngest Daughter is 9 and while she is starting to push back with us a little its not to bad as she has 2 older siblings and can see that we don't let them get away with to much so she does know the limits and boundaries of what she can and cannot get away with.
my advice would be, make sure you and your partner are on the same page when it comes to discipline, show a united front towards her crap else its only going to get worse.
This doesn't mean you both have to act like prison guards but when she does go into one you both need to be firm.
hope things get better for you and good luck.