Piece of music about infiltrating Republican Party (5)
I believe the answer is:
rondo
'piece of music' is the definition.
(type of musical composition)
'about infiltrating republican party' is the wordplay.
'about' becomes 'on' (as in a book 'on' a certain topic).
'infiltrating' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'party' becomes 'do' (do is a kind of party).
'r'+'do'='rdo'
'on' placed into 'rdo' is 'RONDO'.
(Other definitions for rondo that I've seen before include "Donor (anag) - piece of music" , "Piece in the form of a sonata's last movement, say" , "Repetitive musical form" , "Musical work" , "Piece of music with recurring theme" .)