Fed up, made the mistake of having it postponed (8)
I believe the answer is:
deferred
'postponed' is the definition.
('defer' can be a synonym of 'postpone')
'fed up made the mistake of having it' is the wordplay.
I cannot quite see how this works, but
an anagram of 'fed' is 'def' which is within the answer.
'mistake' could be 'erred' (I've seen this before) and 'erred' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for deferred that I've seen before include "Put back" , "Put off to a later time" , "Postponed" .)