If you imagine the new clue for '1a' completing the loop, it will be a normal clue to the entry at 12a (currently lacking a clue) with an extra definition of the entry at 1a (currently provided by 'buttocks'). The clue labelled 12a is a normal clue to the entry at 25d with an extra definition of the entry at 12a.
With this sort of puzzle there are usually several competitors who send in a clue for the wrong word, quite understandably in my view!