Jif 73, the unmodified answer to 21 across is hidden in the 2nd, 3rd, 4th and 5th words of the clue. It's a very obscure word but when changed becomes the definition in 15a.
I am still stuck on 10 across and 3d. I assume the definition for 10a is in 9a but the only word I can find means either "pipes" or "horses' anuses"! I can't make head nor tail of 3d.