Demand Yankee auditor's time of reckoning? (3,4)
I believe the answer is:
tax year
'time of reckoning?' is the definition.
(time for working out taxes)
'demand yankee auditor's' is the wordplay.
'demand' becomes 'tax' (I've seen this before).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'auditor' becomes 'ear' (audit literally means to hear).
'tax'+'y'+'ear'='TAX YEAR'
(Other definitions for tax year that I've seen before include "I start in spring" , "I finish on 5/4" , "It runs in the UK from the beginning of April" .)