Part of speech opens with ambition (4)
I believe the answer is:
hope
'ambition' is the definition.
(thesaurus)
'part of speech opens with' is the wordplay.
'part of' indicates a hidden word.
'HOPE' is hidden within 'speech opens with'.
(Other definitions for hope that I've seen before include "Cross your fingers" , "It springs eternal, it's said" , "Feeling of optimism" , "Bob --, US comedian" , "English-born US comedian, d. 2003" .)