Old diary found in one busy Yankee vintner's study (8)
I believe the answer is:
oenology
'vintner's study' is the definition.
(study of wine)
'old diary found in one busy yankee' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'diary' becomes 'log' (log can mean a record or diary).
'found in' is an insertion indicator.
'busy' is an anagram indicator.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'one' is an anagram of 'eno'.
'eno'+'y'='enoy'
'log' placed inside 'enoy' is 'enology'.
'o'+'enology'='OENOLOGY'
(Other definitions for oenology that I've seen before include "Wine knowledge" , "The science of wines" , "The study of wine making" , "The art of wine making" .)