Unused open fire permit (6)
I believe the answer is:
fallow
'unused' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'open fire permit' is the wordplay.
'open' suggests taking the first letters (take the opening letter).
'permit' becomes 'allow' (synonyms).
The first letter of 'fire' is 'f'.
'f'+'allow'='FALLOW'
Can you help me to learn more?
(Other definitions for fallow that I've seen before include "(Of fields) left unsown" , "Left unploughed and unseeded" , "(Of land) left unsown" , "Inactive" , "idle" .)