Roll stuffed with old chicken (7)
I believe the answer is:
rooster
'roll stuffed with old chicken' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'roll stuffed with old chicken' is the wordplay.
'roll' becomes 'roster' (synonyms).
'stuffed with' indicates putting letters inside (I've seen this in other clues).
'old chicken' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'roster' enclosing 'o' is 'ROOSTER'.
Can you help me to learn more?
(Other definitions for rooster that I've seen before include "Adult male chicken" , "American cock" , "Male domestic fowl in USA" , "male with comb" , "So retro (anag.)" .)