its not possible to clone a leaf, plus if you did mange to pull it off then all you would have is 1 leaf and nothing more as their would be no stem to grow so you would end up with just 1 leaf for as long as you kept it alive,
here is another option open to you though, if their to small to clone from and you do want to keep the strain around they why not go with a reveg, all you do is flower the plant as normal, but then when you harvest just leave the smallest amount of growth on the plant, so the small tiny buds or a few small lower inner branches, leave these on the plant after harvest then put the plant back under a veg schedule, id use 24-0 or 20-4 until the plant is back in veg then use the schedule you want to use,
i reveg often and it gives you a monster cropped plant which means many many more branches and some really crazy growth, harvest next time round is increased by anything from 20 to 40%
a reveg does take time how ever, after you harvest the plant needs to be fed a weak veg nutrient feed about 1/4 strength, then put plant under 20-4 or 24-0. for the first week you wont notice any new growth, week 2 you will notice some strange single bladed leaves, week 3 you will be back in veg and you will have a monster cropped plant which means many many branches, you will have branches growing out of branches and you will have more clones than you know what to do with, instead of having a small xmas tree shaped plant you will have a big bush that looks like its been under a scrog, the yield will be a lot bigger after a reveg, once its back in veg just veg as long as you like then flip back to 12-12 for flower,
ive revegged the same plant 3 or 4 times with no problems what so ever, all that happens is you get a monster cropped plant with many branches,