Unfortunately, see mishap causing stress (9)
I believe the answer is:
emphasise
'stress' is the definition.
('stress'->'emphasize' is in my internal thesaurus and 'ize'->'ise')
'unfortunately see mishap' is the wordplay.
'unfortunately' indicates anagramming the letters.
'see'+'mishap'='seemishap'
'seemishap' with letters rearranged gives 'EMPHASISE'.
'causing' is the link.
(Other definitions for emphasise that I've seen before include "Stress, underline" , "Give extra weight to, stress" , "Place stress on" , "'Stress, accentuate (9)'" , "Mashes pie (anag.)" .)