Roughly struck in the head, rebel falls (7)
I believe the answer is:
cascade
'falls' is the definition.
(synonyms)
'roughly struck in the head rebel' is the wordplay.
'roughly' becomes 'ca' (abbreviation for circa).
'in the head' indicates taking the first letters.
'rebel' becomes 'cade' (15th-century rebel Jack Cade).
The initial letter of 'struck' is 's'.
'ca'+'s'+'cade'='CASCADE'
(Other definitions for cascade that I've seen before include "Rush" , "Shower" , "Series of small waterfalls" , "Outpouring" , "Tumble down (like water)" .)