fortunate happenstance (11)
I believe the answer is:
serendipity
'fortunate happenstance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for serendipity that I've seen before include "Ability to make fortunate discoveries by accident" , "Faculty of making beneficial discoveries by accident" , "Beneficial happenstance" , "Lucky happenstance" , "The accidental discovery of good fortune" .)