Alongside is an animal crossing river (7)
I believe the answer is:
abreast
'alongside' is the definition.
(abreast means side by side or at the side of)
'an animal crossing river' is the wordplay.
'an' becomes 'a'.
'animal' becomes 'beast' (synonyms).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'beast' placed around 'r' is 'breast'.
'a'+'breast'='ABREAST'
'is' is the link.
(Other definitions for abreast that I've seen before include "shoulder to shoulder" , "Level or keeping pace with" , "acquainted" , "Level in line" , "Familiar" .)