Time will cause betrayal (7)
I believe the answer is:
treason
'betrayal' is the definition.
(I know that betrayal can be written as treason)
'time will cause' is the wordplay.
I cannot really see how this works, but
'time' could be 't' (abbreviation) and 't' is found in the answer.
'cause' could be 'reason' (I've seen this before) and 'reason' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for treason that I've seen before include "Crime of disloyalty" , "Perfidy" , "Deliberate act of betrayal" , "attempting to overthrow the king" , "Offence against state" .)