Reckon teatime's been changed (8)
I believe the answer is:
estimate
'reckon' is the definition.
(I know that reckon can be written as estimate)
'teatime's been changed' is the wordplay.
'been changed' is an anagram indicator.
'teatimes' anagrammed gives 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Reckon value" , "Guess the cost" , "Gauge - approximation" , "Judge the approximate value" , "Assess value" .)