Promise goatherd keeps (4)
I believe the answer is:
oath
'promise' is the definition.
(I know that oath is a type of promise)
'goatherd keeps' is the wordplay.
'keeps' says the answer is hidden in the clue.
'OATH' is hidden in the letters of 'goatherd'.
(Other definitions for oath that I've seen before include "It could be ruddy when minced" , "Strong language" , "A curse or solemn declaration" , "A curse or a vow" , "Vow; profanity" .)