Had happened to have been about to yield to temptation (8)
I believe the answer is:
befallen
'had happened to' is the definition.
I can't judge whether this defines the answer.
'been about to yield to temptation' is the wordplay.
'about' means one lot of letters goes inside another.
'to yield to temptation' becomes 'fall' (I've seen this in another clue).
'been' going around 'fall' is 'BEFALLEN'.
'have' acts as a link.
Can you help me to learn more?
(Other definitions for befallen that I've seen before include "(Having) happened to (someone)" , "Came to pass" .)