Finished on time, that's plain (5)
I believe the answer is:
overt
'plain' is the definition.
(both can mean visible or obvious)
'finished on time' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'over'+'t'='OVERT'
'that's' is the link.
(Other definitions for overt that I've seen before include "Trove (anag.)" , "Open,up front" , "Plainly apparent" , "Plain to see - manifest" , "Not concealed - public" .)