Change in time unconvincing (5)
I believe the answer is:
tweak
'change' is the definition.
(I've seen this before)
'time unconvincing' is the wordplay.
'time' becomes 't' (abbreviation).
'unconvincing' becomes 'weak' (I've seen this in another clue).
't'+'weak'='TWEAK'
'in' acts as a link.
(Other definitions for tweak that I've seen before include "Twitch" , "Slightly shift" , "Nip" , "Squeeze with the fingers" , "Make minor adjustment" .)