Hit leads to a duck? Jolly good! (6)
I believe the answer is:
whacko
'jolly good' is the definition.
(I've seen this before)
'hit leads to a duck?' is the wordplay.
'hit' becomes 'whack' (whacking is a kind of hitting**).
'leads' says to put letters next to each other.
'to a duck?' becomes 'o'.
'whack'+'o'='WHACKO'
(Other definitions for whacko that I've seen before include "Mad" , "Jolly good" , "Eccentric" , "nuts" .)