It's advisable to leave nothing to a dunderhead (4)
I believe the answer is:
good
'it's advisable' is the definition.
(thesaurus)
'leave nothing to a dunderhead' is the wordplay.
'leave' becomes 'go' (to go is to depart or leave).
'nothing' becomes 'O' (looks like zero - 0).
'to a dunderhead' becomes 'd' (head letter of 'dunder').
'go'+'o'+'d'='GOOD'
'to' is the link.
(Other definitions for good that I've seen before include "Enjoyable -- moral" , "and 22 Down and 14 Across: Christmas carol" , "Carol character from 1 as of old" , "promoting welfare" , "Having desirable qualities" .)