Bonnie's companion is a Scottish runner (5)
I believe the answer is:
clyde
'runner' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for clyde that I've seen before include "One running between banks" , "Bonnie's partner" , "Bonnie and --, outlaws" , "Criminal" , "bank robber" .)