Great activity finished by force (9)
I believe the answer is:
overdrive
'great activity' is the definition.
(I know that overdrive is a type of great activity)
'finished by force' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'by' says to put letters next to each other.
'force' becomes 'drive' (synonyms).
'over'+'drive'='OVERDRIVE'
(Other definitions for overdrive that I've seen before include "State of increased activity" , "Find extra gear" , "Furious activity" , "High gear; excessive activity" , "Work too hard" .)