One very loud Yankee becoming doubtful (4)
I believe the answer is:
iffy
'doubtful' is the definition.
(thesaurus)
'one very loud yankee' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'very loud' becomes 'ff' (musical abbreviation).
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'i'+'ff'+'y'='IFFY'
'becoming' acts as a link.
(Other definitions for iffy that I've seen before include "Doubtful (colloq.)" , "Uncertain (colloq.)" , "Informally dubious or uncertain" , "Problematic" , "Colloquially uncertain or doubtful" .)