Hello druh6sm,
Not claiming to be an expert, but I'll try to help....
16a definition is the last two words, but not referring to brain grey matter, more something geological (I think!). The "lacking quiet" refers to removing a common crossword single letter (musical usage), otherwise "sadly" indicates anagram of first words. I only got this one with the help of the crossers!
21a definition is first word. "one" is common single letter to be included in a synonym for "reticent type".
Hope that helps, with apologies for any stupid mistakes I have made. Bit nervous that normally replies come from people who I can see are very experienced at this game!