Well, this is embarrassing. Re kidneys, I had a 5 letter word that fits with kidneys, the singular of which is rare (according to Chambers) but the singular is 3 letters. I understand the anagram, know what the spare letter must be and can construct a word that exists (but not in Chambers 2014). It seems to exist in combination with eg blood vessels indicating 'of the kidney'.
If I'm right I think I'll take it as a signal to go and do the Azed instead!