Speed too much and go over the limit (7)
I believe the answer is:
overrun
'speed' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'too much and go over the limit' is the wordplay.
'too much' becomes 'over' (over can mean to too great an extent).
'and' says to put letters next to each other.
'go over the limit' becomes 'run' (I can't explain this - if you can you should give a lot more credence to this answer).
'over'+'run'='OVERRUN'
Can you help me to learn more?
(Other definitions for overrun that I've seen before include "Invade conclusively - extend beyond limit" , "Invade - exceed time limit" , "Invade in great numbers" , "Exceed (allowed time/cost)" , "(Programme) take too long; defeat easily" .)