Unexpected opportunity (6)
I believe the answer is:
chance
I believe this clue is a double definition.
'unexpected' is the first definition.
(thesaurus)
'opportunity' is the second definition.
('chance' can be a synonym of 'opportunity')
(Other definitions for chance that I've seen before include "The unpredictable element of existence" , "Probability" , "Liability" , "Accidental - try" , "Luck or accident" .)