Bears running at exterior, side by side (7)
I believe the answer is:
abreast
'side by side' is the definition.
(synonyms)
'bears running at exterior' is the wordplay.
'running' is an anagram indicator.
'exterior' is an insertion indicator.
'bears' anagrammed gives 'breas'.
'breas' going within 'at' is 'ABREAST'.
(Other definitions for abreast that I've seen before include "Level or keeping pace with" , "Familiar" , "Side by side - in the picture" , "Level in line" , "acquainted" .)