Having the I from THIRD HAND I managed the down clue and another across and then did two more, guessing wildly, but the answers are clearly right and fit - goodness knows why!
I now have
19 down: Soubriquet's essence in letter written up (7)
Letters: E - - - H - T
so I jump to the conclusion that it must be EPITHET, but if so, why, oh why!
And then that leaves only a few more to do ...