Cook did stuffing for chicken out of sight (6)
I believe the answer is:
hidden
'out of sight' is the definition.
(synonyms)
'cook did stuffing for chicken' is the wordplay.
'cook' is an anagram indicator.
'stuffing for' means one lot of letters goes inside another.
'chicken' becomes 'hen' (hen is a kind of chicken).
'did' is an anagram of 'idd'.
'idd' inserted into 'hen' is 'HIDDEN'.
(Other definitions for hidden that I've seen before include "Obscure" , "Not accessible to view" , "Unseen" , "Covert, obscured" , "Concealed, kept secret" .)