Pedran, I've learnt to not worry about the 'why' so much as what the answer is. Untether is definitely right as in meaning 'release'. Unter (hunter minus the h) is the horse part of clue 30 and 'the' is the article, but instead of 'the' going round 'unter' it's the opposite. ie 'unter' going round 'the'. If anyone out there has a better explanation i'm happy to hear it.
Thanks for your contributions. I will go with UNTETHER (I had ?N?E?H?R and no other answer made any sense. I like to know the why, as the more I understand about cryptic conventions, the better chance I will have of finishing a crossword!