Fall back in common mutual defence (6)
I believe the answer is:
autumn
'fall' is the definition.
(I know that fall can be written as autumn)
'back in common mutual defence' is the wordplay.
'back' says the letters should be written in reverse.
'in' indicates a hidden word.
'nmutua' can be found hidden inside 'common mutual defence'.
'nmutua' back-to-front is 'AUTUMN'.
(Other definitions for autumn that I've seen before include "Subject of 11 [ODE]" , "Fall (over the pond?)" , "addressed by Keats" , "Season, has its music too (Keats)" , "Season of falling leaves" .)