Time's out at one on assessment (10)
I believe the answer is:
estimation
'assessment' is the definition.
(I know that estimation is a type of assessment)
'time's out at one on' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'one' becomes 'i' (Roman numeral).
'times' is an anagram of 'estim'.
'estim'+'at'+'i'+'on'='ESTIMATION'
(Other definitions for estimation that I've seen before include "Reckoning of value" , "Opinion" , "Guess" , "Judgment" , "View" .)