Fellow candidate, company during marathon (7,4)
I believe the answer is:
running mate
'fellow candidate' is the definition.
'running mate' can be an answer for 'candidate' (running mate is a kind of candidate). I'm unsure of the 'fellow' bit.
'company during marathon' is the wordplay.
I cannot quite understand how this works, but
'during' could be 'in' (e.g. during February means in February) and 'in' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for running mate that I've seen before include "Candidate" , "Political partner" .)