I got there in the end, but am stumped as to how to parse two of them.
In 15, I think you need to change something which is somehow defined at the start of the clue into the definition given at the end, but what is that 'something'?
17 I knew immediately, thanks to the crossers, but cannot parse. I cannot account for letters 6 and 7 (and possibly 3).
Also, a question about 25. The two letters in the definition are regularly used in solutions, is it common to see them in a definition as an abbreviation, or is this an attempt at misdirection? A double bluff as it were as I assumed it might not be the abbreviation.
Thanks to all the contributers here, really informative.
P.S. I am posting this via my mobile and it is VERY unresponsive when typing the message. Is that normal? I think my other posts were from my PC.