Is only half of it unwise rubbish? (5)
I believe the answer is:
trash
'rubbish?' is the definition.
(I know that rubbish can be written as trash)
'is only half of it unwise' is the wordplay.
'is only half of' suggests halving the letters.
'unwise' becomes 'rash'.
'it' cut in half is 't'.
't'+'rash'='TRASH'
(Other definitions for trash that I've seen before include "Refuse; destroy" , "Worthless items" , "Rubbish, litter" , "Take the tops from < rhubarb" , "Drivel" .)