Promise to use bad language (5)
I believe the answer is:
swear
This clue is a double definition.
'promise' is the first definition.
(thesaurus)
'use bad language' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for swear that I've seen before include "Affirm" , "Vow; blaspheme" , "Promise on oath" , "Use bad language" , "Emit oath or take vow" .)