Sound like a farm animal - a duck? Something in a pen? (4)
I believe the answer is:
oink
'sound like a farm animal' is the definition.
'a duck? something in a pen?' is the wordplay.
'a duck?' becomes 'o'.
'something in a pen?' becomes 'ink'.
'o'+'ink'='OINK'
(Other definitions for oink that I've seen before include "Noise made by a pig" , "What you get from pen" , "Pig's grunt" , "Animal sound" , "Cartoon sound of pig" .)