Certainly dismayed, in part (3)
I believe the answer is:
aye
'certainly' is the definition.
(thesaurus)
'dismayed in part' is the wordplay.
'in part' says the answer is hidden in the clue.
'AYE' is hidden within 'dismayed'.
(Other definitions for aye that I've seen before include "Yes vote" , "Yes (archaic)" , "Positive voting response" , "Word of assent" , "Affirmative answer" .)