I find myself clenching my teeth/jaw muscles during the day.
Grinding the teeth together and the consequent pain in the jaw muscles and other muscles too is called bruxism. This clenching of the teeth usually happens at night. More women than men have this condition and it usually starts at about 20 years of age and stops spontaneously at 40.
Also in the morning when I wake up I find I sometimes have sore jaws - so I have been clenching them while asleep.
Continual chenching of the jaw muscles causes destruction of the supportive bone and gums that surround the teeth. It can also lead to TMD or temporomandibular disorder.
I had a conversation about this with a dentist at the Ohio State University College of Dentistry. She said that bruxism is almost always caused by prolonged psychological stress - stress that a person has not learned to cope with. Sometimes this dentist recommends a splint, a bite plate, for bruxers who are doing a lot of damage to their teeth and bone structure. She also said that bruxism is nature's way of telling a person to slow down a bit - reduce the tension in her life.
Can you advise how I could slim my jaw line...
I don't think that it's possible to slim down the jawline. Sometimes the jawline looks less aggressive if you lose weight but that might not be the best option if you are now at the correct weight. Of course if you are finally able to relax your jaw muscles, the muscle fibers might shrink.
Go to the Internet and research "coping strategies for stress" - this might give you a clue on what to do for a more serene sleep at night.
Richard Kiel who played "Jaws" in several movies has successfully dealt with his bruxism. He found that he could reduce his stress level by biting people.