Impasse he cultivated causes stress (9)
I believe the answer is:
emphasise
'stress' is the definition.
(I've seen this before)
'impasse he cultivated' is the wordplay.
'cultivated' indicates anagramming the letters.
'impasse'+'he'='impassehe'
'impassehe' is an anagram of 'EMPHASISE'.
'causes' is the link.
(Other definitions for emphasise that I've seen before include "'Stress, accentuate (9)'" , "Stress, underline" , "Place stress on" , "Give extra importance to" , "Give extra weight to, stress" .)