Movement Republican party guards against (5)
I believe the answer is:
rondo
'movement' is the definition.
(I have seen 'Repetitive movement' mean 'rondo' so perhaps 'movement' could also mean 'rondo')
'republican party guards against' is the wordplay.
'republican' becomes 'R' (abbreviation).
'party' becomes 'do' (do can informally mean a party).
'guards' means one lot of letters goes inside another (inserted letters are guarded).
'against' becomes 'on' (on can mean by or up against).
'r'+'do'='rdo'
'rdo' enclosing 'on' is 'RONDO'.
(Other definitions for rondo that I've seen before include "Musical form with repeated theme" , "Last movement of a sonata or concerto" , "Music with repeated refrains" , "Piece written" , "Donor (anag) - piece of music" .)