I've finished but I'm puzzled by some of the parsing:
17A Article that's first [yep, OK] seen thematically in 8 downs
I have ANT, which could be in the theme word or literally in the theme animal! The letters are also in 8D, but not in sequence. Also, I have a plural word for 8 down, a type of [theme animal] - so why is there a double plural?
17D Second person to be [yep, OK] 18... [18D= TEA] (I have ARE, but what has that to do with 18?)
18D ... or 20, say [I have ECHIDNA], of 26 for builders? [I have MANTLE] (I have TEA for 18D)
21A 7 [I have CAST ASIDE] of 24 [I have WANTON] worried about... (I have ETA)
Apart from the answers spelling out the theme animal clockwise, I can't make any sense of these clues and would not have 'solved' them (I don't really feel I have at all).