American Satan devouring Republican alongside (7)
I believe the answer is:
abreast
'alongside' is the definition.
(abreast means side by side or at the side of)
'american satan devouring republican' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'satan' becomes 'beast' (both can mean the Devil).
'devouring' is an insertion indicator.
'republican' becomes 'r' (abbreviation).
'beast' placed around 'r' is 'breast'.
'a'+'breast'='ABREAST'
(Other definitions for abreast that I've seen before include "acquainted" , "Well-informed" , "to be up to date" , "shoulder to shoulder" , "Side by side; keeping pace" .)