Fall back in marathon, mutually (6)
I believe the answer is:
autumn
'fall' is the definition.
('autumn' can be a synonym of 'fall')
'back in marathon mutually' is the wordplay.
'back' is a reversal indicator.
'in' indicates a hidden word.
'nmutua' can be found hidden inside 'marathon mutually'.
'nmutua' backwards is 'AUTUMN'.
(Other definitions for autumn that I've seen before include "part of year?" , "Subject of 11 [ODE]" , "Fall for Mrs Obama" , "Harvest season" , "Season of falling leaves" .)