Prevents American former soldiers arresting Republican (6)
I believe the answer is:
averts
'prevents' is the definition.
(averting is a kind of preventing)
'american former soldiers arresting republican' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'former soldiers' becomes 'vets' (I've seen this in another clue).
'arresting' means one lot of letters goes inside another.
'republican' becomes 'R' (political abbreviation).
'vets' placed around 'r' is 'verts'.
'a'+'verts'='AVERTS'
(Other definitions for averts that I've seen before include "Deflects, heads off" , "Turns away or causes to turn away" , "Heads off, prevents from happening" , "Turns aside" , "Stops" .)