Out to lunch, Sue has mint relish (10)
I believe the answer is:
enthusiasm
'relish' is the definition.
(thesaurus)
'out to lunch sue has mint' is the wordplay.
'out to lunch' indicates anagramming the letters (out to lunch can mean mad).
'sue'+'has'+'mint'='suehasmint'
'suehasmint' with letters rearranged gives 'ENTHUSIASM'.
(Other definitions for enthusiasm that I've seen before include "Ardour" , "Gusto; fervour" , "Intense and lively interest" , "spirit?" , "Avidity" .)