Guarantee you'll fly off the handle and be mistrustful about (8)
I believe the answer is:
warranty
'guarantee' is the definition.
(I know that guarantee can be written as warranty)
'be mistrustful about' is the wordplay.
'be' becomes 'rant' (I can't explain this - if you can you should give a lot more credence to this answer).
'mistrustful' becomes 'wary' (I've seen this in another clue).
'about' is an insertion indicator.
'rant' going into 'wary' is 'WARRANTY'.
'you'll fly off the handle and' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for warranty that I've seen before include "Contract" , "cover provided" , "Undertaking that covers sale" , "Guarantee given to a purchaser" , "Quality guarantee" .)