Is it possible that clenching can make chewing surfaces indent deeper exposing darker enamel or dentin? outer and inner surface seem to be sharper?why

Yes: Without a doubt. Especially if there is any acid reflux. Acid and clenching or grinding can destroy your teeth. Clenching and an acidic diet can be a problem. If you let it go and see the dark dentin you are in trouble because the dentin isn't anywhere near as hard as the enamel and will wear away even faster.

Abrasion: Your teeth are being worn away from your habit. See your dentist about making a night guard to help protect your teeth. It will be less expensive in the long run.

Yes: Clenching is associated also with grinding side to side. This wears the outer enamel exposing the 2nd later of the tooth (dentin). You are reshaping the tooth.

Clenching: If you are clenching you are probably also grinding.this will care excessive wear of the enamel, making the darker underlying dentin more visible. Have your Dentist check your occlusion, and discuss the need for a bite guard and/or Orthodontic treatment and/or restorative treatment.