Plan mutual piece to be put up before winter (6)
I believe the answer is:
autumn
'winter' is the definition.
(thesaurus)
'plan mutual piece to be put up' is the wordplay.
'piece' says the answer is hidden in the clue.
'to be put up' says the letters should be written in reverse.
'nmutua' is hidden in the letters of 'plan mutual'.
'nmutua' written backwards gives 'AUTUMN'.
'before' is the link.
(Other definitions for autumn that I've seen before include "part of year?" , "Time of year" , "'O wild West Wind, thou breath of . . . . . . 's being' (Shelley)" , "Fall in North America" , "American call it the fall" .)