Finished before time, that’s obvious (5)
I believe the answer is:
overt
'obvious' is the definition.
('overt' can be similar in meaning to 'obvious')
'finished before time' is the wordplay.
'finished' becomes 'over' ('over' can be similar in meaning to 'finished'**).
'before' is a charade indicator (letters next to each other).
'time' becomes 't'.
'over'+'t'='OVERT'
'that's' acts as a link.
(Other definitions for overt that I've seen before include "Obvious, patent" , "Trove (anag.)" , "not disguised" , "Not locked away" , "Done or shown openly" .)