Alongside 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.
'river' becomes 'r' (abbreviation for river).
'beast' going around 'r' is 'breast'.
'a'+'breast'='ABREAST'
(Other definitions for abreast that I've seen before include "Well-informed" , "to be up to date" , "Level or keeping pace with" , "Level in line" , "shoulder to shoulder" .)