Three left.
30a Root around knowing one won't be around long (6) - - Y - L -
20a Gentle 21 - no way! (6) - - N - E -
17d Element of symbollically backing biblical city (9) R - - - E - I - -
21 is NURSERY.
I can fit these with 30 MAYFLY, 20 TENDER, 17 RUTHENIUM, but I can't see any connection or word play for any of them. Thanks a lot.