Tax haven short of capital — officer set upon (7)
I believe the answer is:
assault
'set upon' is the definition.
(set upon can mean to attack)
'tax haven short of capital officer' is the wordplay.
'tax haven' becomes 'Nassau' (capital of the Bahamas, often called a tax haven).
'short of capital' means to remove the first letter (the initial or capital is removed).
'officer' becomes 'lt' (abbreviation for lieutenant).
'nassau' with its initial letter taken off is 'assau'.
'assau'+'lt'='ASSAULT'
(Other definitions for assault that I've seen before include "Attack associated with" , "Attack violently" , "Lash" , "Sudden violent attack" , "Violent attack in action or words" .)