Loose faun sent off (8)
I believe the answer is:
unfasten
'loose' is the definition.
(thesaurus)
'faun sent off' is the wordplay.
'off' indicates anagramming the letters.
'faun'+'sent'='faunsent'
'faunsent' anagrammed gives 'UNFASTEN'.
(Other definitions for unfasten that I've seen before include "Nun's fate (anag.)" , "Let go" , "Undo" , "tie? No" , "Unlock" .)