Post in time? Wrong! (8)
I believe the answer is:
mistaken
'wrong' is the definition.
(similar in meaning)
'post in time?' is the wordplay.
'post' becomes 'stake' (synonyms).
'in' indicates putting letters inside.
'time?' becomes 'min' (short for minute).
'stake' put within 'min' is 'MISTAKEN'.
(Other definitions for mistaken that I've seen before include "Wrong (in judgement)" , "Wrong in opinion, judgment etc." , "In error in mankiest way" , "Wrong, incorrect" , "Wrongly understood" .)