Fit in marathon -- after beer, oddly! (6)
I believe the answer is:
belong
'fit in' is the definition.
(to belong somewhere is to fit in there)
'marathon after beer oddly' is the wordplay.
'marathon' becomes 'long' (marathon can mean long or lengthy).
'after' means one lot of letters go next to another.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'beer' are 'be'.
'long' after 'be' is 'BELONG'.
(Other definitions for belong that I've seen before include "to have a home" , "Appertain" , "Be the property of" , "'Be part of, fit in (6)'" , "Be attached" .)