After a fast time, nothing is slow! (5)
I believe the answer is:
lento
'is slow' is the definition.
(similar in meaning)
'after a fast time nothing' is the wordplay.
'after' means one lot of letters go next to another.
'a fast time' becomes 'lent' (I've seen this in another clue).
'nothing' becomes 'O' (O resembles 0).
'lent' next to 'o' is 'LENTO'.
(Other definitions for lento that I've seen before include "(Of tempo) slow" , "Slow (mus.)" , "(When playing music) slowly" , "How one scores slowly" , "Slow passage" .)