Gladly given permit to remain unproductive (3,6)
I believe the answer is:
lie fallow
'to remain unproductive' is the definition.
'gladly given permit' is the wordplay.
'gladly' becomes 'lief' ('lief' can be a synonym of 'gladly').
'given' is a charade indicator (letters next to each other).
'permit' becomes 'allow' ('allow' can be a synonym of 'permit').
'lief'+'allow'='LIE FALLOW'
(Other definitions for lie fallow that I've seen before include "remain inactive" , "Remain cropless" .)