Heartless old jailer producing dead duck (6)
I believe the answer is:
turkey
'dead duck' is the definition.
(both can mean a failure or disappointment)
'heartless old jailer' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'old jailer' becomes 'turnkey' (I've seen this before).
'turnkey' with its centre taken out is 'TURKEY'.
'producing' acts as a link.
(Other definitions for turkey that I've seen before include "complete failure" , "one gobbles" , "Christmas dinner?" , "which is found in more than one continent" , "A failed play or a seasonal bird" .)