... beside a marathon team (9)
I believe the answer is:
alongside
'beside' is the definition.
(thesaurus)
'a marathon team' is the wordplay.
'a marathon' becomes 'along' (I can't explain this - if you can you should give a lot more credence to this answer).
'team' becomes 'side' (both can mean a sports team).
'along'+'side'='ALONGSIDE'
Can you help me to learn more?
(Other definitions for alongside that I've seen before include "Aligned so as to be next to, as ship moored to pier" , "Parallel to" , "Next to, another boat perhaps" , "Lying next to, as a boat might be" , "Lying close to" .)