Break one's word (9)
I believe the answer is:
hyphenate
'break one's word' is the definition.
(hyphenating a word breaks it in two)
This is the entire clue.
(Other definitions for hyphenate that I've seen before include "with short line" , "the line in three-legged race" , "Make a dash" , "Join two words or syllables with short dash" , "Join two words with horizontal dash" .)