Enthusiastic about nothing but duck (5)
I believe the answer is:
avoid
'duck' is the definition.
(thesaurus)
'enthusiastic about nothing' is the wordplay.
'enthusiastic' becomes 'avid' ('avid' can be similar in meaning to 'enthusiastic').
'about' indicates putting letters inside.
'nothing' becomes 'o' (looks like zero - 0).
'avid' going around 'o' is 'AVOID'.
'but' acts as a link.
(Other definitions for avoid that I've seen before include "Shirk" , "Fend off" , "Desist" , "Shy away from" , "Sidestep" .)