Make too few runs, maybe creating stress (10)
I believe the answer is:
underscore
'stress' is the definition.
(underscoring is a kind of stressing)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for underscore that I've seen before include "Stress" , "It's beneath character to highlight" , "Emphasise" , "19 ?" .)