Goatherd shows such promise (4)
I believe the answer is:
oath
'promise' is the definition.
(thesaurus)
'goatherd shows' is the wordplay.
'shows' says the answer is hidden in the clue.
'OATH' can be found hidden inside 'goatherd'.
'such' is the link.
(Other definitions for oath that I've seen before include "Curse or solemn promise" , "It's taken in court" , "Commitment to tell the truth" , "It could be ruddy when minced" , "Solemn vow" .)