First of fields set aside, left unploughed (6)
I believe the answer is:
fallow
'left unploughed' is the definition.
(I've seen this before)
'first of fields set aside' is the wordplay.
'first of' suggests taking the first letters.
'set aside' becomes 'allow' ('allow' can be a synonym of 'set aside').
The initial letter of 'fields' is 'f'.
'f'+'allow'='FALLOW'
(Other definitions for fallow that I've seen before include "Left unploughed and unseeded" , "Inactive" , "(Of land) left unsown" , "(Of fields) left unsown" , "idle" .)