Censor's pet duck? (4)
I believe the answer is:
cato
'censor's' is the definition.
(ancient Roman censor)
'pet duck?' is the wordplay.
'pet' becomes 'cat' (cats are often kept as pets).
'duck?' becomes 'o' (resembles zero - 'duck' in cricket).
'cat'+'o'='CATO'
(Other definitions for cato that I've seen before include "classical writer" , "Roman statesman" , "Ancient Roman censor" , "Senator" , "Marcus Porcius - , Roman politician, censor" .)