hi geo0939 -I hadn't heard of it either - the compiler of this crossword is notorious for selecting obscure definitions. I found this in Chambers - Helot (as well as meaning serf etc) a plant or animal living symbiotically with another in a subject relationship (biol)
Many thanks for HELOT it was the last clue I needed and I agree that the author likes to pick obscure clues, but as you say you generally know they are correct as he tends to go word for word with Chambers. Thanks again