Emphasis on a syllable (6)
I believe the answer is:
stress
'emphasis on a syllable' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stress that I've seen before include "possible cause of fracture?" , "Emphasise the strain" , "State mental or emotional strain" , "Lay emphasis on - it's a strain" , "'Worry, pressure (6)'" .)