Duck is put back in more dietaries (5)
I believe the answer is:
eider
'duck' is the definition.
(eider is a type of duck)
'put back in more dietaries' is the wordplay.
'put back' is a reversal indicator.
'in' indicates a hidden word.
'redie' can be found hidden inside 'more dietaries'.
'redie' reversed gives 'EIDER'.
'is' is the link.
(Other definitions for eider that I've seen before include "Large sea duck" , "Down-producing duck" , "Sea duck with soft down" , "Duck noted for down" , "Variety of duck" .)