Cause of delay, if rejected, gets the Guardian in terrible mess (12)
I believe the answer is:
filibusterer
'cause of delay if rejected gets' is the definition.
I can't tell whether this defines the answer.
'guardian in terrible mess' is the wordplay.
I cannot really understand how this works, but
'guardian' could be 'us' (the crossword setter and their Guardian colleagues) and 'us' is found within the answer.
an anagram of 'terrible' is 'libterer' which is present in the remaining letters.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Another definition for filibusterer that I've seen is " One speaking at length to block legislation".)