What kind of deficiency?

I had something similar happen to me (also in the second week of flowering), and everything indicated it was due to a deficiency in calcium and magnesium, so I applied CalMag and the plants are recovering. Apparently, using LED lights that have a more complete light spectrum makes the plant's photosynthetic activity more intense and requires a boost of those two nutrients (all of them, but particularly keep an eye on those two).

I’d guess nutrient lock out flush and let her dry up and feed her with a full feed

High Marihuano, This is manganese deficiency in the early stage. If caught early, manganese deficiency can usually be corrected by flushing with 6pH water and adding a light 6pH nutrient solution. It’s also wise to make sure there isn’t an overabundance of iron, since that can hinder manganese absorption. If it doesn’t clear up in about a week, consider using manganese chelate. Sadly, if it’s reared its ugly head during blooming, there’s some serious pruning that needs to be done to get rid of affected areas. Just remember to not do it all at once, as suddenly snipping away all that foliage can cause undue stress on the plants. Goodluck!

Wish you would put some info in your diary about what nutes you're giving and what pH level you're maintaining... This looks to be the start of perhaps a calcium deficiency - but the cause of that could be a couple of things... it could be you're giving it too much nitrogen which will cause a lock out and you'd need to flush... or it could be your pH is out of range (in soil, this needs to be 6.0-6.2 for flowering). Just don't have enough information to give you a more concrete answer! Good luck!