Is it permissible to iron 1 wrinkle/crease that is on the cuff of my pants during the 9 days?
Shulchan Aruch forbids ironing from Rosh Chodesh until after the fast. While there are various interpretations for the Hebrew word gihutz, Rav Moshe Feinstein confirms that the activity which we call ironing with an iron is included in the prohibition to iron clothes or to wear ironed clothes during the nine days.
The fact that it is one crease on the cuff of the pants is not a valid reason to permit it. Most likely these custom developed so that we train our behavior to act in a fashion consistent with how we are expected to feel in our hearts. A person mourning the loss of a person usually is not focused on ironing a single crease in the cuff of their slacks. The Shulchan Aruch has given us guidelines so that we can be aware of how we should be feeling about the loss of the Beis Hamikdash.
For many it is difficult to find the appropriate way to really feel the pain of the tragedy of the churban to the extent that the behaviors mentioned in Shulchan Aruch come to us naturally. Still, let us attempt to follow the rules of the Shulchan Aruch meticulously in the hope that it will illicit the correct emotions and avodas halev.
 או”ח תקנ”א ס”ג.
 אג”מ או”ח ח”ג סימן פ”ג.