Speed and ecstasy's under a hundred pounds (7)
I believe the answer is:
crushes
'pounds' is the definition.
The definition and answer can be both acts as well as being plural nouns.
Perhaps you can see an association between them that I don't see?
'speed and ecstasy's under a hundred' is the wordplay.
'speed' becomes 'rush' (synonyms).
'and' means one lot of letters go next to another.
'ecstasy's' becomes 'E's' (informal term for the drug).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'a hundred' becomes 'C' (Roman numeral).
'rush' next to 'es' is 'rushes'.
'rushes' put after 'c' is 'CRUSHES'.
Can you help me to learn more?