Reckon tea-time's changed (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'tea-time's changed' is the wordplay.
'changed' indicates an anagram.
'tea'+'times'='teatimes'
'teatimes' with letters rearranged gives 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "'Judge, calculate (8)'" , "Judgment, appraisal" , "Judge to be probable" , "Approximate calculation" , "Judge the approximate cost of" .)