One party line becomes a fetish (4)
I believe the answer is:
idol
'a fetish' is the definition.
(I've seen this in another clue)
'one party line' is the wordplay.
'one' becomes 'i' (Roman numeral).
'party' becomes 'do' (do can informally mean a party).
'line' becomes 'l' (used when specifying particular lines from a poem).
'i'+'do'+'l'='IDOL'
'becomes' is the link.
(Other definitions for idol that I've seen before include "Favourite" , "loved one" , "Figure of worship" , "One much admired" , "Image of a god (not icon)" .)