Now I've finally got all the bars sorted out, hopefully I can finish the last four clues...with a little help...
29a Sort of rich gloss I neglected (5) P???A
I think the misprint is an E making RICE, and there is a type of rice that fits the letter pattern, but I can see where "gloss I neglected" fits in. I've tried adding an L(eft) to the my answer but it doesn't lead back to "gloss".
19a They're used to cut what's left of fingernails mostly damaged after treatment finally fails (6) - F???M?S
15d Rows perhaps upset head protecting second of teachers (4) ??ET
20d Powdery substances blighted parsley - but not completely (6)
It looks like it's an anagram/blighted of "parsle(y) not completely" but it doesn't lead to anything that would fit a definition.
One or all of these might have a single letter misprint.