Ready on time? Nonsense! (5)
I believe the answer is:
tripe
'nonsense' is the definition.
(informal term for nonsense or false information)
'ready on time?' is the wordplay.
'ready' becomes 'ripe' (similar in meaning).
'on' means one lot of letters go next to another.
'time?' becomes 't' (abbreviation).
'ripe' after 't' is 'TRIPE'.
(Other definitions for tripe that I've seen before include "Junk; food" , "Swallowed" , "Something to eat" , "Lining of the stomach of a cow as food" , "Rubbish; offal" .)