Looking good, thanks everyone. As Jack says, some of them are pretty obvious when you see the answers.
So, just two remaining:-
Main element or fastener [6]
- T - P - RNow with an extra letter..
Kitchen with supersonic cook-up [10]
- E - C - S - I - NI have decided that the answer to the second clue is "PERCUSSION".
Purely because it's an anagram of "supersonic", and also fits your letters. I have no idea what it has to do with kitchens, though.
Yes! Kitchens have kettles in them, and a kettle drum is a percussion instrument!
I too thought percussion, but the clue can't be that tenuous, can it?
Rounders-player reverses the summit [8]
BACKSTOPI still can't see how that was worked out. What's the summit bit?
What a state to be in if I jingle! [4]
FIJIFiji fits, but is it a state?