What deficiency?

Hey there! I agree with Scrubbyjimbob, more info could be helpful. Based on the symptoms displayed, it looks like your plant might be experiencing a nitrogen or potassium deficiency. The yellowing leaves and brown spots are key indicators. I'd recommend checking your nutrient mix and possibly adding a balanced fertilizer that includes these elements. Additionally, make sure your pH levels are in the optimal range (6.0-7.0) to ensure nutrient uptake. The pot seems a little small, so your plant could also be roobound. Consider repotting into a larger one (if you're not growing an autoflower that is) to give the roots more space to grow. Best of luck Cheers, D.

More info or a diary or pics of more than just the leaf in question would help.... If this is just a leaf or two at the bottom of tall plants don't stress, bottom leaves get ugly over time- poor lighting and the first leaves a plant will self prune or cannibalize.