Idle autumn: nothing before onset of winter (6)
I believe the answer is:
fallow
'idle' is the definition.
(I've seen this before)
'autumn nothing before onset of winter' is the wordplay.
'autumn' becomes 'fall' (synonyms).
'nothing' becomes 'o' (looks like zero - 0).
'before' says to put letters next to each other.
'onset of' suggests taking the first letters.
The initial letter of 'winter' is 'w'.
'fall'+'o'+'w'='FALLOW'
(Other definitions for fallow that I've seen before include "(Of land) left unsown" , "Inactive" , "(Of fields) left unsown" , "Left unploughed and unseeded" , "Undeveloped but potentially useful" .)