Bears at assembly, side by side (7)
I believe the answer is:
abreast
'side by side' is the definition.
(synonyms)
'bears at assembly' is the wordplay.
'assembly' indicates anagramming the letters.
'bears'+'at'='bearsat'
'bearsat' anagrammed gives 'ABREAST'.
(Other definitions for abreast that I've seen before include "Side by side - in the picture" , "to be up to date" , "On a level together" , "shoulder to shoulder" , "Well-informed" .)